v3.26.3
Shareholder Report
 12 Months Ended
Jul. 31, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSR
Amendment Flag false
Registrant Name FIRST TRUST EXCHANGE-TRADED ALPHADEX FUND
Entity Central Index Key 0001383496
Entity Investment Company Type N-1A
Document Period End Date Jul. 31, 2026
C000047995 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Large Cap Core AlphaDEX® Fund
Class Name First Trust Large Cap Core AlphaDEX® Fund
Trading Symbol FEX
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Large Cap Core AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FEX. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FEX
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Large Cap Core AlphaDEX® Fund $64 0.57%
Expenses Paid, Amount $ 64
Expense Ratio, Percent 0.57%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 23.01% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Index, which returned 19.56% for the same Period.
During the Period, investments in the Industrials sector received the greatest allocation of any sector with an average weight of 17.1%, and contributed 4.5% to the Fund’s overall return. With an average weight of 15.1%, investments in the Information Technology sector contributed 7.9% to the Fund’s overall return, which was the greatest contribution of any sector. With an average weight of 4.4%, investments in the Communication Services sector contributed -0.5% to the Fund’s overall return, which was the most negative contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Large Cap Core AlphaDEX® Fund 23.01% 10.83% 12.67%
Nasdaq AlphaDEX Large Cap CoreTM Index 23.75% 11.49% 13.36%
Nasdaq US 500 Large CapTM Index 17.80% 12.35% 15.04%
S&P 500® Index 19.56% 12.86% 15.08%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FEX for more recent performance information.
Net Assets $ 1,565,948,975
Holdings Count | Holding 377
Advisory Fees Paid, Amount $ 7,167,515
Investment Company Portfolio Turnover 92.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,565,948,975
Total number of portfolio holdings 377
Total advisory fee paid $7,167,515
Portfolio turnover rate 92%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 0.6%
Accenture PLC, Class A 0.6%
PayPal Holdings, Inc. 0.6%
Regeneron Pharmaceuticals, Inc. 0.6%
ConocoPhillips 0.5%
Global Payments, Inc. 0.5%
Fidelity National Information Services, Inc. 0.5%
EOG Resources, Inc. 0.5%
Travelers (The) Cos., Inc. 0.5%
AT&T, Inc. 0.5%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 0.6%
Accenture PLC, Class A 0.6%
PayPal Holdings, Inc. 0.6%
Regeneron Pharmaceuticals, Inc. 0.6%
ConocoPhillips 0.5%
Global Payments, Inc. 0.5%
Fidelity National Information Services, Inc. 0.5%
EOG Resources, Inc. 0.5%
Travelers (The) Cos., Inc. 0.5%
AT&T, Inc. 0.5%
C000047996 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Mid Cap Core AlphaDEX® Fund
Class Name First Trust Mid Cap Core AlphaDEX® Fund
Trading Symbol FNX
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Mid Cap Core AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FNX. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FNX
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Mid Cap Core AlphaDEX® Fund $64(1) 0.57%(1)
​(1)
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
Expenses Paid, Amount $ 64 [1]
Expense Ratio, Percent 0.57% [1]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 23.25% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P MidCap 400® Index, which returned 20.92% for the same Period.
During the Period, investments in the Industrials sector received the greatest allocation of any sector, with an average weight of 20.6%, which contributed 4.8% to Fund’s overall return, which was the greatest of any sector. With an average weight of 4.1%, investments in the Consumer Staples sector contributed -0.3% to the Fund’s overall return, which was the most negative return contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Mid Cap Core AlphaDEX® Fund 23.25% 8.86% 11.72%
Nasdaq AlphaDEX Mid Cap CoreTM Index 24.01% 9.54% 12.43%
Nasdaq US 600 Mid CapTM Index 21.51% 6.75% 10.78%
S&P MidCap 400® Index 20.92% 8.47% 10.91%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FNX for more recent performance information.
Net Assets $ 1,374,486,934
Holdings Count | Holding 452
Advisory Fees Paid, Amount $ 6,226,984
Investment Company Portfolio Turnover 100.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,374,486,934
Total number of portfolio holdings 452
Total advisory fee paid $6,226,984
Portfolio turnover rate 100%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
EPAM Systems, Inc. 0.5%
Lithia Motors, Inc. 0.5%
HF Sinclair Corp. 0.5%
Genpact Ltd. 0.5%
SM Energy Co. 0.5%
Ovintiv, Inc. 0.4%
ADT, Inc. 0.4%
Permian Resources Corp., Class A 0.4%
CF Industries Holdings, Inc. 0.4%
Asbury Automotive Group, Inc. 0.4%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
EPAM Systems, Inc. 0.5%
Lithia Motors, Inc. 0.5%
HF Sinclair Corp. 0.5%
Genpact Ltd. 0.5%
SM Energy Co. 0.5%
Ovintiv, Inc. 0.4%
ADT, Inc. 0.4%
Permian Resources Corp., Class A 0.4%
CF Industries Holdings, Inc. 0.4%
Asbury Automotive Group, Inc. 0.4%
C000047997 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Small Cap Core AlphaDEX® Fund
Class Name First Trust Small Cap Core AlphaDEX® Fund
Trading Symbol FYX
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Small Cap Core AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FYX. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FYX
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Small Cap Core AlphaDEX® Fund $70 0.57%
Expenses Paid, Amount $ 70
Expense Ratio, Percent 0.57%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 45.58% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P SmallCap 600® Index, which returned 33.64% for the same Period.
During the Period, investments in the Financials sector received the greatest allocation of any sector, with an average weight of 19.5%, which contributed 6.2% to the Fund’s overall return. With an average weight of 13.7%, investments in the Health Care sector contributed 8.1% to the Fund’s overall return, which was the greatest of any sector. No sector represented a negative contribution to the Fund’s overall return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Small Cap Core AlphaDEX® Fund 45.58% 10.49% 12.39%
Nasdaq AlphaDEX Small Cap CoreTM Index 46.60% 11.23% 13.14%
Nasdaq US 700 Small CapTM Index 36.82% 7.73% 11.70%
S&P SmallCap 600® Index 33.64% 7.48% 10.75%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FYX for more recent performance information.
Net Assets $ 1,376,674,389
Holdings Count | Holding 527
Advisory Fees Paid, Amount $ 5,256,295
Investment Company Portfolio Turnover 102.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,376,674,389
Total number of portfolio holdings 527
Total advisory fee paid $5,256,295
Portfolio turnover rate 102%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
CBIZ, Inc. 0.5%
Everforth, Inc. 0.5%
PBF Energy, Inc., Class A 0.5%
Par Pacific Holdings, Inc. 0.5%
CONMED Corp. 0.5%
Dorian LPG Ltd. 0.4%
DXC Technology Co. 0.4%
International Seaways, Inc. 0.4%
Korn Ferry 0.4%
Acadian Asset Management, Inc. 0.4%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
CBIZ, Inc. 0.5%
Everforth, Inc. 0.5%
PBF Energy, Inc., Class A 0.5%
Par Pacific Holdings, Inc. 0.5%
CONMED Corp. 0.5%
Dorian LPG Ltd. 0.4%
DXC Technology Co. 0.4%
International Seaways, Inc. 0.4%
Korn Ferry 0.4%
Acadian Asset Management, Inc. 0.4%
C000047998 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Large Cap Value AlphaDEX® Fund
Class Name First Trust Large Cap Value AlphaDEX® Fund
Trading Symbol FTA
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Large Cap Value AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FTA. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FTA
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Large Cap Value AlphaDEX® Fund $66 0.57%
Expenses Paid, Amount $ 66
Expense Ratio, Percent 0.57%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 30.11% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Value Index, which returned 19.71% for the same Period.
During the Period, investments in the Financials sector received the greatest allocation of any sector, with an average weight of 21.4% and contributed 4.9% to the Fund’s overall return. With an average weight of 11.4%, investments in the Health Care sector contributed 5.7% to the Fund’s overall return, which was the greatest of any sector. No sector contributed a negative return to the Fund’s overall return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Large Cap Value AlphaDEX® Fund 30.11% 11.16% 11.34%
Nasdaq AlphaDEX Large Cap ValueTM Index 30.95% 11.84% 12.05%
Nasdaq US 500 Large Cap ValueTM Index 28.48% 11.46% 12.56%
S&P 500® Index 19.56% 12.86% 15.08%
S&P 500® Value Index 19.71% 11.57% 11.84%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FTA for more recent performance information.
Net Assets $ 1,398,022,182
Holdings Count | Holding 189
Advisory Fees Paid, Amount $ 6,147,891
Investment Company Portfolio Turnover 83.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,398,022,182
Total number of portfolio holdings 189
Total advisory fee paid $6,147,891
Portfolio turnover rate 83%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 1.2%
Accenture PLC, Class A 1.1%
PayPal Holdings, Inc. 1.1%
ConocoPhillips 1.0%
Global Payments, Inc. 1.0%
General Motors Co. 1.0%
Fidelity National Information Services, Inc. 1.0%
EOG Resources, Inc. 1.0%
Travelers (The) Cos., Inc. 1.0%
AT&T, Inc. 1.0%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 1.2%
Accenture PLC, Class A 1.1%
PayPal Holdings, Inc. 1.1%
ConocoPhillips 1.0%
Global Payments, Inc. 1.0%
General Motors Co. 1.0%
Fidelity National Information Services, Inc. 1.0%
EOG Resources, Inc. 1.0%
Travelers (The) Cos., Inc. 1.0%
AT&T, Inc. 1.0%
C000047999 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Large Cap Growth AlphaDEX® Fund
Class Name First Trust Large Cap Growth AlphaDEX® Fund
Trading Symbol FTC
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Large Cap Growth AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FTC. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FTC
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Large Cap Growth AlphaDEX® Fund $61 0.57%
Expenses Paid, Amount $ 61
Expense Ratio, Percent 0.57%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 12.88% during the 12 months ended July 31, 2026. The Fund underperformed its benchmark, the S&P 500® Growth Index, which returned 19.42% for the same Period.
During the Period, investments in the Information Technology sector received the greatest allocation of any sector, with an average weight of 25.9%, and contributed 9.9% to the Fund’s overall return. Investments in the Communication Services, with an average weight of 4.8%, contributed -1.2% to the Fund’s overall return, which was the most negative contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Large Cap Growth AlphaDEX® Fund 12.88% 9.80% 13.59%
Nasdaq AlphaDEX Large Cap GrowthTM Index 13.55% 10.45% 14.28%
Nasdaq US 500 Large Cap GrowthTM Index 13.98% 12.62% 16.76%
S&P 500® Index 19.56% 12.86% 15.08%
S&P 500® Growth Index 19.42% 13.32% 17.40%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FTC for more recent performance information.
Net Assets $ 1,318,673,505
Holdings Count | Holding 189
Advisory Fees Paid, Amount $ 6,343,316
Investment Company Portfolio Turnover 113.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,318,673,505
Total number of portfolio holdings 189
Total advisory fee paid $6,343,316
Portfolio turnover rate 113%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Incyte Corp. 1.0%
Fortinet, Inc. 1.0%
Datadog, Inc., Class A 1.0%
eBay, Inc. 1.0%
Interactive Brokers Group, Inc., Class A 1.0%
Morgan Stanley 1.0%
EMCOR Group, Inc. 0.9%
Cincinnati Financial Corp. 0.9%
United Rentals, Inc. 0.9%
Ross Stores, Inc. 0.9%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Incyte Corp. 1.0%
Fortinet, Inc. 1.0%
Datadog, Inc., Class A 1.0%
eBay, Inc. 1.0%
Interactive Brokers Group, Inc., Class A 1.0%
Morgan Stanley 1.0%
EMCOR Group, Inc. 0.9%
Cincinnati Financial Corp. 0.9%
United Rentals, Inc. 0.9%
Ross Stores, Inc. 0.9%
C000048000 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Multi Cap Value AlphaDEX® Fund
Class Name First Trust Multi Cap Value AlphaDEX® Fund
Trading Symbol FAB
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Multi Cap Value AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FAB. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FAB
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Multi Cap Value AlphaDEX® Fund $75(1) 0.65%(1)
​(1)
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
Expenses Paid, Amount $ 75 [2]
Expense Ratio, Percent 0.65% [2]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 30.17% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P Composite 1500® Value Index, which returned 20.19% for the same Period.
During the Period, the average allocation to the Financials sector was 22.7%, the greatest of any sector, and contributed 5.8% to the Fund’s overall return. With an average weight of 11.5%, investments in the Energy sector contributed 6.0% to the Fund’s overall return, the greatest contribution to the Fund’s overall return. No sector represented a negative contribution to the Fund’s overall return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Multi Cap Value AlphaDEX® Fund 30.17% 10.20% 10.83%
Nasdaq AlphaDEX Multi Cap ValueTM Index 31.11% 10.95% 11.62%
Nasdaq US Multi Cap ValueTM Index 28.66% 11.13% 12.33%
S&P Composite 1500® Index 19.97% 12.48% 14.72%
S&P Composite 1500® Value Index 20.19% 11.36% 11.73%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FAB for more recent performance information.
Net Assets $ 168,465,133
Holdings Count | Holding 676
Advisory Fees Paid, Amount $ 649,600
Investment Company Portfolio Turnover 85.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $168,465,133
Total number of portfolio holdings 676
Total advisory fee paid $649,600
Portfolio turnover rate 85%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 0.6%
Accenture PLC, Class A 0.6%
PayPal Holdings, Inc. 0.6%
Global Payments, Inc. 0.5%
ConocoPhillips 0.5%
General Motors Co. 0.5%
Fidelity National Information Services, Inc. 0.5%
EOG Resources, Inc. 0.5%
Travelers (The) Cos., Inc. 0.5%
AT&T, Inc. 0.5%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 0.6%
Accenture PLC, Class A 0.6%
PayPal Holdings, Inc. 0.6%
Global Payments, Inc. 0.5%
ConocoPhillips 0.5%
General Motors Co. 0.5%
Fidelity National Information Services, Inc. 0.5%
EOG Resources, Inc. 0.5%
Travelers (The) Cos., Inc. 0.5%
AT&T, Inc. 0.5%
C000048001 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Multi Cap Growth AlphaDEX® Fund
Class Name First Trust Multi Cap Growth AlphaDEX® Fund
Trading Symbol FAD
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Multi Cap Growth AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FAD. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FAD
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Multi Cap Growth AlphaDEX® Fund $66(1) 0.59%(1)
​(1)
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
Expenses Paid, Amount $ 66 [3]
Expense Ratio, Percent 0.59% [3]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 22.09% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P Composite 1500® Growth Index, which returned 19.78% for the same Period.
During the Period, the Fund’s greatest allocation was to the Industrials sector. The Industrials sector received an average allocation of 25.2% and contributed 6.4% to the Fund’s overall return. With an average weight of 21.1%, investments in the Information Technology sector contributed 9.0% to the Fund’s overall return, the greatest contribution of the Fund’s overall return. With an average weight of 1.7%, investments in the Utilities sector contributed -0.3% to the Fund’s overall return, the most negative contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Multi Cap Growth AlphaDEX® Fund 22.09% 9.34% 13.57%
Nasdaq AlphaDEX Multi Cap GrowthTM Index 22.86% 10.05% 14.33%
Nasdaq US Multi Cap GrowthTM Index 14.38% 11.92% 16.18%
S&P Composite 1500® Index 19.97% 12.48% 14.72%
S&P Composite 1500® Growth Index 19.78% 12.86% 16.87%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FAD for more recent performance information.
Net Assets $ 577,445,793
Holdings Count | Holding 678
Advisory Fees Paid, Amount $ 2,079,056
Investment Company Portfolio Turnover 106.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $577,445,793
Total number of portfolio holdings 678
Total advisory fee paid $2,079,056
Portfolio turnover rate 106%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Incyte Corp. 0.5%
Fortinet, Inc. 0.5%
Datadog, Inc., Class A 0.5%
eBay, Inc. 0.5%
Interactive Brokers Group, Inc., Class A 0.5%
Morgan Stanley 0.5%
EMCOR Group, Inc. 0.5%
Cincinnati Financial Corp. 0.5%
United Rentals, Inc. 0.5%
Ross Stores, Inc. 0.5%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Incyte Corp. 0.5%
Fortinet, Inc. 0.5%
Datadog, Inc., Class A 0.5%
eBay, Inc. 0.5%
Interactive Brokers Group, Inc., Class A 0.5%
Morgan Stanley 0.5%
EMCOR Group, Inc. 0.5%
Cincinnati Financial Corp. 0.5%
United Rentals, Inc. 0.5%
Ross Stores, Inc. 0.5%
C000048896 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Consumer Discretionary AlphaDEX® Fund
Class Name First Trust Consumer Discretionary AlphaDEX® Fund
Trading Symbol FXD
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Consumer Discretionary AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXD. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXD
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Consumer Discretionary AlphaDEX® Fund $67 0.64%
Expenses Paid, Amount $ 67
Expense Ratio, Percent 0.64%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 8.85% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Consumer Discretionary Index, which returned 7.52% for the same Period.
During the Period, investments in the Specialty Retail industry received the greatest allocation of any industry with an average weight of 18.4% and contributed 2.7% to the Fund’s overall return, which was the greatest return contribution of any industry. With an average weight of 1.9%, investments in the Distributors industry contributed –0.6% to the Fund’s overall return, which was the most negative return contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Consumer Discretionary AlphaDEX® Fund 8.85% 3.55% 7.77%
StrataQuant® Consumer Discretionary Index 9.50% 4.16% 8.43%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Consumer Discretionary Index 7.52% 6.81% 12.62%
Russell 1000® Consumer Discretionary Index 2.33% 5.76% 12.68%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXD for more recent performance information.
Net Assets $ 269,173,641
Holdings Count | Holding 114
Advisory Fees Paid, Amount $ 1,464,185
Investment Company Portfolio Turnover 85.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $269,173,641
Total number of portfolio holdings 114
Total advisory fee paid $1,464,185
Portfolio turnover rate 85%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Fox Corp., Class A 1.7%
Dillard’s, Inc., Class A 1.7%
Mattel, Inc. 1.6%
Lyft, Inc., Class A 1.6%
Gap (The), Inc. 1.6%
SharkNinja, Inc. 1.6%
Lithia Motors, Inc. 1.6%
Macy’s, Inc. 1.6%
Lululemon Athletica, Inc. 1.6%
Sirius XM Holdings, Inc. 1.5%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Fox Corp., Class A 1.7%
Dillard’s, Inc., Class A 1.7%
Mattel, Inc. 1.6%
Lyft, Inc., Class A 1.6%
Gap (The), Inc. 1.6%
SharkNinja, Inc. 1.6%
Lithia Motors, Inc. 1.6%
Macy’s, Inc. 1.6%
Lululemon Athletica, Inc. 1.6%
Sirius XM Holdings, Inc. 1.5%
C000048897 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Consumer Staples AlphaDEX® Fund
Class Name First Trust Consumer Staples AlphaDEX® Fund
Trading Symbol FXG
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Consumer Staples AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXG. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXG
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Consumer Staples AlphaDEX® Fund $66 0.64%
Expenses Paid, Amount $ 66
Expense Ratio, Percent 0.64%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 4.71% during the 12 months ended July 31, 2026. The Fund underperformed its benchmark, the S&P 500® Consumer Staples Index, which returned 10.32% for the same Period.
During the Period, investments in the Food Products industry received the greatest allocation of any industry with an average weight of 48.6% and contributed –1.3% to the Fund’s overall return, which was the most negative return contribution of any industry. With an average weight of 18.1%, investments in the Beverages industry contributed 2.4% to the Fund’s overall return, which was the greatest return contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Consumer Staples AlphaDEX® Fund 4.71% 4.75% 4.60%
StrataQuant® Consumer Staples Index 4.69% 5.23% 5.18%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Consumer Staples Index 10.32% 7.69% 8.22%
Russell 1000® Consumer Staples Index 12.63% 6.73% 6.64%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXG for more recent performance information.
Net Assets $ 229,274,437
Holdings Count | Holding 42
Advisory Fees Paid, Amount $ 1,267,678
Investment Company Portfolio Turnover 95.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $229,274,437
Total number of portfolio holdings 42
Total advisory fee paid $1,267,678
Portfolio turnover rate 95%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
JBS N.V., Class A 4.8%
Smithfield Foods, Inc. 4.4%
Ingredion, Inc. 4.3%
General Mills, Inc. 4.2%
McCormick & Co., Inc. 4.1%
Freshpet, Inc. 4.1%
Seaboard Corp. 4.0%
Pilgrim’s Pride Corp. 4.0%
Casey’s General Stores, Inc. 3.6%
Post Holdings, Inc. 3.4%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
JBS N.V., Class A 4.8%
Smithfield Foods, Inc. 4.4%
Ingredion, Inc. 4.3%
General Mills, Inc. 4.2%
McCormick & Co., Inc. 4.1%
Freshpet, Inc. 4.1%
Seaboard Corp. 4.0%
Pilgrim’s Pride Corp. 4.0%
Casey’s General Stores, Inc. 3.6%
Post Holdings, Inc. 3.4%
C000048898 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Energy AlphaDEX® Fund
Class Name First Trust Energy AlphaDEX® Fund
Trading Symbol FXN
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Energy AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXN. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXN
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Energy AlphaDEX® Fund $75 0.62%
Expenses Paid, Amount $ 75
Expense Ratio, Percent 0.62%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 43.00% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Energy Index, which returned 41.17% for the same Period.
During the Period, investments in the Oil, Gas & Consumable Fuels industry received the greatest allocation of any industry with an average weight of 80.4% and contributed 32.7% to the Fund’s overall return, which was the greatest return contribution of any industry. With an average weight of 0.1%, investments in the Electrical Equipment industry contributed –0.2% to the Fund’s overall return, which was the most negative contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Energy AlphaDEX® Fund 43.00% 19.30% 6.72%
StrataQuant® Energy Index 44.02% 20.13% 7.43%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Energy Index 41.17% 23.99% 10.16%
Russell 1000® Energy Index 40.15% 22.92% 9.82%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXN for more recent performance information.
Net Assets $ 379,138,406
Holdings Count | Holding 40
Advisory Fees Paid, Amount $ 2,620,187
Investment Company Portfolio Turnover 77.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $379,138,406
Total number of portfolio holdings 40
Total advisory fee paid $2,620,187
Portfolio turnover rate 77%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments of the Fund.
Top Ten Holdings
HF Sinclair Corp. 5.7%
APA Corp. 5.0%
Devon Energy Corp. 4.8%
Range Resources Corp. 4.7%
Expand Energy Corp. 4.5%
Antero Resources Corp. 4.5%
EQT Corp. 4.4%
Chord Energy Corp. 3.7%
Ovintiv, Inc. 3.6%
ConocoPhillips 3.5%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
HF Sinclair Corp. 5.7%
APA Corp. 5.0%
Devon Energy Corp. 4.8%
Range Resources Corp. 4.7%
Expand Energy Corp. 4.5%
Antero Resources Corp. 4.5%
EQT Corp. 4.4%
Chord Energy Corp. 3.7%
Ovintiv, Inc. 3.6%
ConocoPhillips 3.5%
C000048899 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Financials AlphaDEX® Fund
Class Name First Trust Financials AlphaDEX® Fund
Trading Symbol FXO
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Financials AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXO. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXO
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Financials AlphaDEX® Fund $66 0.61%
Expenses Paid, Amount $ 66
Expense Ratio, Percent 0.61%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 17.93% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Financials Index, which returned 10.52% for the same Period.
During the Period, investments in the Insurance industry received the greatest allocation of any industry with an average weight of 34.2% and contributed 6.2% to the Fund’s overall return. With an average weight of 27.6%, investments in the Banks industry contributed 8.0% to the Fund’s overall return, which was the greatest return contribution of any industry. No industry had a negative contribution to overall Fund return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Financials AlphaDEX® Fund 17.93% 11.37% 13.17%
StrataQuant® Financials Index 18.73% 12.07% 13.92%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Financials Index 10.52% 11.27% 13.68%
Russell 1000® Financials Index 12.45% 13.05% 15.40%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXO for more recent performance information.
Net Assets $ 1,128,685,877
Holdings Count | Holding 106
Advisory Fees Paid, Amount $ 8,561,297
Investment Company Portfolio Turnover 70.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,128,685,877
Total number of portfolio holdings 106
Total advisory fee paid $8,561,297
Portfolio turnover rate 70%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
MarketAxess Holdings, Inc. 2.3%
Lincoln National Corp. 2.0%
FactSet Research Systems, Inc. 1.8%
Travelers (The) Cos., Inc. 1.8%
Allstate (The) Corp. 1.7%
First American Financial Corp. 1.7%
Central BanCo, Inc. 1.7%
Hartford Insurance Group (The), Inc. 1.7%
MGIC Investment Corp. 1.7%
American International Group, Inc. 1.7%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
MarketAxess Holdings, Inc. 2.3%
Lincoln National Corp. 2.0%
FactSet Research Systems, Inc. 1.8%
Travelers (The) Cos., Inc. 1.8%
Allstate (The) Corp. 1.7%
First American Financial Corp. 1.7%
Central BanCo, Inc. 1.7%
Hartford Insurance Group (The), Inc. 1.7%
MGIC Investment Corp. 1.7%
American International Group, Inc. 1.7%
C000048900 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Health Care AlphaDEX® Fund
Class Name First Trust Health Care AlphaDEX® Fund
Trading Symbol FXH
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Health Care AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXH. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXH
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Health Care AlphaDEX® Fund $71 0.62%
Expenses Paid, Amount $ 71
Expense Ratio, Percent 0.62%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 29.63% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Health Care Index, which returned 26.92% for the same Period.
During the Period, investments in the Biotechnology industry received the greatest allocation of any industry with an average weight of 26.4% and contributed 10.1% to the Fund’s overall return. With an average weight of 25.0%, investments in the Health Care Providers & Services industry contributed 12.4% to the Fund’s overall return, which was the greatest return contribution of any industry. With an average weight of 2.0%, investments in the Health Care Technology industry contributed –0.9% to the Fund’s overall return, which was the most negative return contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Health Care AlphaDEX® Fund 29.63% 1.19% 7.72%
StrataQuant® Health Care Index 30.52% 1.81% 8.41%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Health Care Index 26.92% 6.02% 9.97%
Russell 1000® Health Care Index 26.59% 5.25% 9.94%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXH for more recent performance information.
Net Assets $ 1,054,221,358
Holdings Count | Holding 75
Advisory Fees Paid, Amount $ 4,569,417
Investment Company Portfolio Turnover 124.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,054,221,358
Total number of portfolio holdings 75
Total advisory fee paid $4,569,417
Portfolio turnover rate 124%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Regeneron Pharmaceuticals, Inc. 2.8%
Universal Health Services, Inc., Class B 2.6%
Solventum Corp. 2.5%
Boston Scientific Corp. 2.5%
Zimmer Biomet Holdings, Inc. 2.5%
Guardant Health, Inc. 2.4%
DaVita, Inc. 2.4%
Pfizer, Inc. 2.4%
Tenet Healthcare Corp. 2.3%
Cigna Group (The) 2.3%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Regeneron Pharmaceuticals, Inc. 2.8%
Universal Health Services, Inc., Class B 2.6%
Solventum Corp. 2.5%
Boston Scientific Corp. 2.5%
Zimmer Biomet Holdings, Inc. 2.5%
Guardant Health, Inc. 2.4%
DaVita, Inc. 2.4%
Pfizer, Inc. 2.4%
Tenet Healthcare Corp. 2.3%
Cigna Group (The) 2.3%
C000048901 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Industrials/Producer Durables AlphaDEX® Fund
Class Name First Trust Industrials/Producer Durables AlphaDEX® Fund
Trading Symbol FXR
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Industrials/Producer Durables AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXR. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXR
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Industrials/Producer Durables AlphaDEX® Fund $67 0.62%
Expenses Paid, Amount $ 67
Expense Ratio, Percent 0.62%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 15.16% during the 12 months ended July 31, 2026. The Fund underperformed its benchmark, the S&P 500® Industrials Index, which returned 19.86% for the same Period.
During the Period, investments in the Machinery industry received the greatest allocation of any industry with an average weight of 14.6% and contributed 3.6% to the Fund’s overall return, which was the greatest return contribution of any industry. With an average weight of 8.3%, investments in the Financial Services industry contributed –1.6% to the Fund’s overall return, which was the most negative return contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Industrials/Producer Durables AlphaDEX® Fund 15.16% 9.04% 12.72%
StrataQuant® Industrials Index 15.77% 9.74% 13.48%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Industrials Index 19.86% 13.49% 13.67%
Russell 1000® Industrials Index 14.52% 8.98% 11.89%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXR for more recent performance information.
Net Assets $ 741,607,839
Holdings Count | Holding 157
Advisory Fees Paid, Amount $ 5,684,364
Investment Company Portfolio Turnover 74.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $741,607,839
Total number of portfolio holdings 157
Total advisory fee paid $5,684,364
Portfolio turnover rate 74%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Accenture PLC, Class A 1.5%
PayPal Holdings, Inc. 1.5%
Genpact Ltd. 1.4%
ADT, Inc. 1.3%
Fidelity National Information Services, Inc. 1.3%
Vontier Corp. 1.2%
Fiserv, Inc. 1.2%
ExlService Holdings, Inc. 1.2%
Graphic Packaging Holding Co. 1.1%
Mohawk Industries, Inc. 1.1%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Accenture PLC, Class A 1.5%
PayPal Holdings, Inc. 1.5%
Genpact Ltd. 1.4%
ADT, Inc. 1.3%
Fidelity National Information Services, Inc. 1.3%
Vontier Corp. 1.2%
Fiserv, Inc. 1.2%
ExlService Holdings, Inc. 1.2%
Graphic Packaging Holding Co. 1.1%
Mohawk Industries, Inc. 1.1%
C000048902 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Materials AlphaDEX® Fund
Class Name First Trust Materials AlphaDEX® Fund
Trading Symbol FXZ
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Materials AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXZ. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXZ
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Materials AlphaDEX® Fund $75 0.64%
Expenses Paid, Amount $ 75
Expense Ratio, Percent 0.64%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 35.49% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Materials Index, which returned 15.32% for the same Period.
During the Period, investments in the Chemicals industry received the greatest allocation of any industry, with an average weight of 40.9%, and contributed 4.7% to the Fund’s overall return. With an average weight of 37.1%, investments in the Metals & Mining industry contributed 20.8% to the Fund’s overall return, which was the greatest return contribution of any industry. No industry had a negative contribution to overall Fund return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Materials AlphaDEX® Fund 35.49% 7.58% 10.08%
StrataQuant® Materials Index 36.33% 8.28% 10.83%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Materials Index 15.32% 5.53% 9.64%
Russell 1000® Basic Materials Index 25.52% 7.91% 10.77%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXZ for more recent performance information.
Net Assets $ 368,177,305
Holdings Count | Holding 41
Advisory Fees Paid, Amount $ 1,483,638
Investment Company Portfolio Turnover 73.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $368,177,305
Total number of portfolio holdings 41
Total advisory fee paid $1,483,638
Portfolio turnover rate 73%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
CF Industries Holdings, Inc. 5.6%
Steel Dynamics, Inc. 5.3%
Newmont Corp. 4.9%
Anglogold Ashanti PLC 4.8%
Coeur Mining, Inc. 4.4%
Alcoa Corp. 4.2%
Nucor Corp. 3.9%
Mueller Industries, Inc. 3.7%
Eastman Chemical Co. 3.6%
Mosaic (The) Co. 3.6%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
CF Industries Holdings, Inc. 5.6%
Steel Dynamics, Inc. 5.3%
Newmont Corp. 4.9%
Anglogold Ashanti PLC 4.8%
Coeur Mining, Inc. 4.4%
Alcoa Corp. 4.2%
Nucor Corp. 3.9%
Mueller Industries, Inc. 3.7%
Eastman Chemical Co. 3.6%
Mosaic (The) Co. 3.6%
C000048903 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Technology AlphaDEX® Fund
Class Name First Trust Technology AlphaDEX® Fund
Trading Symbol FXL
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Technology AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXL. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXL
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Technology AlphaDEX® Fund $69 0.61%
Expenses Paid, Amount $ 69
Expense Ratio, Percent 0.61%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 24.65% during the 12 months ended July 31, 2026. The Fund underperformed its benchmark, the S&P 500® Information Technology Index, which returned 26.22% for the same Period.
During the Period, investments in the Semiconductors & Semiconductor Equipment industry received the greatest allocation of any industry with an average weight of 29.0% and contributed 23.8% to the Fund’s overall return, which was the greatest return contribution of any industry. With an average weight of 8.3%, investments in the Professional Services industry contributed -2.4% to the Fund’s overall return, which was the most negative return contribution of any industry.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Technology AlphaDEX® Fund 24.65% 9.95% 19.42%
StrataQuant Technology® Index 25.38% 10.55% 20.06%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Information Technology Index 26.22% 20.39% 25.21%
Russell 1000® Technology Index 26.91% 19.59% 25.06%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXL for more recent performance information.
Net Assets $ 2,569,937,525
Holdings Count | Holding 112
Advisory Fees Paid, Amount $ 7,736,247
Investment Company Portfolio Turnover 112.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $2,569,937,525
Total number of portfolio holdings 112
Total advisory fee paid $7,736,247
Portfolio turnover rate 112%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 2.4%
EPAM Systems, Inc. 2.2%
PTC, Inc. 2.0%
Leidos Holdings, Inc. 1.9%
Zoom Communications, Inc. 1.8%
Amdocs Ltd. 1.8%
Adobe, Inc. 1.6%
Dell Technologies, Inc., Class C 1.6%
RingCentral, Inc., Class A 1.4%
Western Digital Corp. 1.4%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Cognizant Technology Solutions Corp., Class A 2.4%
EPAM Systems, Inc. 2.2%
PTC, Inc. 2.0%
Leidos Holdings, Inc. 1.9%
Zoom Communications, Inc. 1.8%
Amdocs Ltd. 1.8%
Adobe, Inc. 1.6%
Dell Technologies, Inc., Class C 1.6%
RingCentral, Inc., Class A 1.4%
Western Digital Corp. 1.4%
C000048904 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Utilities AlphaDEX® Fund
Class Name First Trust Utilities AlphaDEX® Fund
Trading Symbol FXU
Security Exchange Name NYSEArca
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Utilities AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FXU. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FXU
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Utilities AlphaDEX® Fund $65 0.61%
Expenses Paid, Amount $ 65
Expense Ratio, Percent 0.61%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 11.49% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P 500® Utilities Index, which returned 6.41% for the same Period.
During the Period, investments in the Electric Utilities industry received the greatest allocation of any industry with an average weight of 50.3% and contributed 7.8% to the Fund's overall return, which was the greatest return contribution of any industry. No industry had a negative contribution to overall Fund return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Utilities AlphaDEX® Fund 11.49% 12.09% 8.72%
StrataQuant® Utilities Index 12.23% 12.88% 9.45%
Russell 1000® Index 18.94% 12.12% 14.82%
S&P 500® Utilities Index 6.41% 9.41% 8.94%
Russell 1000® Utilities Index 5.60% 9.72% 8.30%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FXU for more recent performance information.
Net Assets $ 831,047,490
Holdings Count | Holding 42
Advisory Fees Paid, Amount $ 6,459,969
Investment Company Portfolio Turnover 42.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $831,047,490
Total number of portfolio holdings 42
Total advisory fee paid $6,459,969
Portfolio turnover rate 42%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
National Fuel Gas Co. 4.5%
UGI Corp. 4.4%
PG&E Corp. 4.4%
AES (The) Corp. 4.2%
Eversource Energy 4.2%
Edison International 4.2%
Consolidated Edison, Inc. 4.2%
Exelon Corp. 4.2%
Essential Utilities, Inc. 3.5%
Duke Energy Corp. 3.4%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
National Fuel Gas Co. 4.5%
UGI Corp. 4.4%
PG&E Corp. 4.4%
AES (The) Corp. 4.2%
Eversource Energy 4.2%
Edison International 4.2%
Consolidated Edison, Inc. 4.2%
Exelon Corp. 4.2%
Essential Utilities, Inc. 3.5%
Duke Energy Corp. 3.4%
C000099059 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Mid Cap Growth AlphaDEX® Fund
Class Name First Trust Mid Cap Growth AlphaDEX® Fund
Trading Symbol FNY
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Mid Cap Growth AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FNY. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FNY
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Mid Cap Growth AlphaDEX® Fund $79(1) 0.71%(1) (2)
​(1)
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
​(2)
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Paid, Amount $ 79 [4]
Expense Ratio, Percent 0.71% [4],[5]
Extraordinary Expenses Footnote [Text Block] Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Excluding Extraordinary Expenses, Percent 0.70%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 21.30% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P MidCap 400® Growth Index, which returned 21.18% for the same Period.
During the Period, investments in the Industrials sector received the greatest allocation of any sector, with an average weight of 26.7%, and contributed 6.3% to the Fund’s overall return, which was the greatest of any sector. Investments in the Consumer Staples sector, with an average weight of 1.7%, contributed -0.8% to Fund’s overall return, which was the most negative contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Mid Cap Growth AlphaDEX® Fund 21.30% 7.03% 12.86%
Nasdaq AlphaDEX Mid Cap GrowthTM Index 22.16% 7.82% 13.69%
Nasdaq US 600 Mid Cap GrowthTM Index 14.88% 3.64% 10.48%
S&P MidCap 400® Growth Index 21.18% 7.57% 10.96%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FNY for more recent performance information.
Net Assets $ 577,587,332
Holdings Count | Holding 227
Advisory Fees Paid, Amount $ 3,450,038
Investment Company Portfolio Turnover 135.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $577,587,332
Total number of portfolio holdings 227
Total advisory fee paid $3,450,038
Portfolio turnover rate 135%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Etsy, Inc. 0.9%
Guardant Health, Inc. 0.9%
PACS Group, Inc. 0.9%
Victoria’s Secret & Co. 0.8%
Amneal Pharmaceuticals, Inc. 0.8%
Enova International, Inc. 0.8%
Corcept Therapeutics, Inc. 0.8%
IES Holdings, Inc. 0.8%
Tutor Perini Corp. 0.8%
Coca-Cola Consolidated, Inc. 0.8%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Etsy, Inc. 0.9%
Guardant Health, Inc. 0.9%
PACS Group, Inc. 0.9%
Victoria’s Secret & Co. 0.8%
Amneal Pharmaceuticals, Inc. 0.8%
Enova International, Inc. 0.8%
Corcept Therapeutics, Inc. 0.8%
IES Holdings, Inc. 0.8%
Tutor Perini Corp. 0.8%
Coca-Cola Consolidated, Inc. 0.8%
C000099060 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Mid Cap Value AlphaDEX® Fund
Class Name First Trust Mid Cap Value AlphaDEX® Fund
Trading Symbol FNK
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Mid Cap Value AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FNK. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FNK
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Mid Cap Value AlphaDEX® Fund $79(1) 0.71%(1) (2)
​(1)
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
​(2)
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Paid, Amount $ 79 [6]
Expense Ratio, Percent 0.71% [6],[7]
Extraordinary Expenses Footnote [Text Block] Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Excluding Extraordinary Expenses, Percent 0.70%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 22.53% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P MidCap 400® Value Index, which returned 20.77% for the same Period.
During the Period, investments in the Financials sector received the greatest allocation of any sector, with an average weight of 24.4%, and contributed 6.2% to the Fund’s overall return, which was the greatest of any sector. Investments in the Consumer Staples sector, with an average weight of 6.0%, contributed -0.1% to the Fund’s overall return, which was the most negative return contribution of any sector.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Mid Cap Value AlphaDEX® Fund 22.53% 9.06% 9.76%
Nasdaq AlphaDEX Mid Cap ValueTM Index 23.46% 9.88% 10.59%
Nasdaq US 600 Mid Cap ValueTM Index 28.24% 9.55% 10.44%
S&P MidCap 400® Value Index 20.77% 9.25% 10.55%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FNK for more recent performance information.
Net Assets $ 228,068,181
Holdings Count | Holding 227
Advisory Fees Paid, Amount $ 1,450,052
Investment Company Portfolio Turnover 104.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $228,068,181
Total number of portfolio holdings 227
Total advisory fee paid $1,450,052
Portfolio turnover rate 104%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
EPAM Systems, Inc. 0.9%
Lithia Motors, Inc. 0.9%
HF Sinclair Corp. 0.9%
Genpact Ltd. 0.9%
SM Energy Co. 0.9%
Ovintiv, Inc. 0.8%
ADT, Inc. 0.8%
Permian Resources Corp., Class A 0.8%
CF Industries Holdings, Inc. 0.8%
Asbury Automotive Group, Inc. 0.8%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
EPAM Systems, Inc. 0.9%
Lithia Motors, Inc. 0.9%
HF Sinclair Corp. 0.9%
Genpact Ltd. 0.9%
SM Energy Co. 0.9%
Ovintiv, Inc. 0.8%
ADT, Inc. 0.8%
Permian Resources Corp., Class A 0.8%
CF Industries Holdings, Inc. 0.8%
Asbury Automotive Group, Inc. 0.8%
C000099061 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Small Cap Growth AlphaDEX® Fund
Class Name First Trust Small Cap Growth AlphaDEX® Fund
Trading Symbol FYC
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Small Cap Growth AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FYC. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FYC
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Small Cap Growth AlphaDEX® Fund $87 0.70%
Expenses Paid, Amount $ 87
Expense Ratio, Percent 0.70%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 48.72% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P SmallCap 600® Growth Index, which returned 30.43% for the same Period.
During the Period, the Health Care sector received the greatest allocation of any sector, with an average weight of 24.0%, and contributed 15.1% to the Fund’s return, the largest positive contribution of any sector. No sector represented a negative contribution to the Fund’s overall return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Small Cap Growth AlphaDEX® Fund 48.72% 11.24% 13.92%
Nasdaq AlphaDEX Small Cap GrowthTM Index 49.94% 12.10% 14.81%
Nasdaq US 700 Small Cap GrowthTM Index 39.41% 7.71% 12.58%
S&P SmallCap 600® Growth Index 30.43% 6.69% 11.13%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FYC for more recent performance information.
Net Assets $ 1,321,993,861
Holdings Count | Holding 266
Advisory Fees Paid, Amount $ 5,962,456
Investment Company Portfolio Turnover 127.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $1,321,993,861
Total number of portfolio holdings 266
Total advisory fee paid $5,962,456
Portfolio turnover rate 127%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
Acadian Asset Management, Inc. 0.8%
Chefs’ Warehouse (The), Inc. 0.8%
RingCentral, Inc., Class A 0.8%
Precigen, Inc. 0.7%
First Advantage Corp. 0.7%
Oscar Health, Inc., Class A 0.7%
AtaiBeckley, Inc. 0.7%
National HealthCare Corp. 0.7%
Liquidia Corp. 0.7%
NeoGenomics, Inc. 0.7%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Acadian Asset Management, Inc. 0.8%
Chefs’ Warehouse (The), Inc. 0.8%
RingCentral, Inc., Class A 0.8%
Precigen, Inc. 0.7%
First Advantage Corp. 0.7%
Oscar Health, Inc., Class A 0.7%
AtaiBeckley, Inc. 0.7%
National HealthCare Corp. 0.7%
Liquidia Corp. 0.7%
NeoGenomics, Inc. 0.7%
C000099062 [Member]  
Shareholder Report [Line Items]  
Fund Name First Trust Small Cap Value AlphaDEX® Fund
Class Name First Trust Small Cap Value AlphaDEX® Fund
Trading Symbol FYT
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about the First Trust Small Cap Value AlphaDEX® Fund (the “Fund”) for the year of August 1, 2025 to July 31, 2026 (the “Period”).
Shareholder Report Annual or Semi-Annual annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at www.ftportfolios.com/fund-documents/etf/FYT. You can also request this information by contacting us at 1-800-621-1675 or info@ftportfolios.com.
Additional Information Phone Number 1-800-621-1675
Additional Information Email info@ftportfolios.com
Additional Information Website www.ftportfolios.com/fund-documents/etf/FYT
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR?
(Based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage
of a $10,000 investment
First Trust Small Cap Value AlphaDEX® Fund $86 0.71%(1)
​(1)
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Paid, Amount $ 86
Expense Ratio, Percent 0.71% [8]
Extraordinary Expenses Footnote [Text Block] Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
Expenses Excluding Extraordinary Expenses, Percent 0.70%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR? WHAT AFFECTED THE FUND’S PERFORMANCE?
The Fund returned 42.37% during the 12 months ended July 31, 2026. The Fund outperformed its benchmark, the S&P SmallCap 600® Value Index, which returned 37.16% for the same Period.
During the Period, investments in the Financials sector received the greatest allocation of any sector with an average weight of 23.6% and contributed 7.3% to the Fund’s overall return. With an average weight of 11.5%, investments in the Energy sector contributed 9.7% to the Fund’s overall return, which was the greatest return contribution of any sector. No sector represented a negative contribution to the Fund’s overall return.
Performance Past Does Not Indicate Future [Text] The Fund’s past performance is not a good predictor of the Fund’s future performance.
Line Graph [Table Text Block]
FUND PERFORMANCE (July 31, 2016 to July 31, 2026)
The performance line graph below shows the performance of a hypothetical $10,000 initial investment in the Fund over a ten-year period (or for the life of the Fund, if shorter). The subsequent account value as of the end of the Period is listed next to the name of the Fund or index, as applicable. The performance table below shows the average annual total returns of the Fund for the past one-, five-, and ten-year periods, as applicable (or for the life of the Fund, if shorter), as of the end of the Period. Both the line graph and performance table compare the Fund’s performance to an appropriate broad-based index and may compare to additional indices reflecting the market segment(s) in which the Fund invests over the same periods.
Investment Performance of $10,000
Fund Performance - Growth of 10K
Average Annual Return [Table Text Block]
Average Annual Total Returns (as of July 31, 2026) 1 Year 5 Year 10 Year
First Trust Small Cap Value AlphaDEX® Fund 42.37% 9.11% 10.68%
Nasdaq AlphaDEX Small Cap ValueTM Index 43.59% 10.01% 11.54%
Nasdaq US 700 Small Cap ValueTM Index 34.02% 7.51% 10.53%
S&P SmallCap 600® Value Index 37.16% 8.14% 10.19%
Russell 3000® Index 19.60% 11.82% 14.56%
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares.
Updated Performance Information Location [Text Block]
Visit  www.ftportfolios.com/etf/FYT for more recent performance information.
Net Assets $ 194,555,801
Holdings Count | Holding 264
Advisory Fees Paid, Amount $ 1,089,872
Investment Company Portfolio Turnover 110.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (As of July 31, 2026)
Fund net assets $194,555,801
Total number of portfolio holdings 264
Total advisory fee paid $1,089,872
Portfolio turnover rate 110%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (As of July 31, 2026)
The tables below show the investment makeup of the Fund, representing the percentage of total investments (excluding repurchase agreements held as collateral for securities lending arrangements) of the Fund.
Top Ten Holdings
CBIZ, Inc. 1.1%
Everforth, Inc. 1.0%
PBF Energy, Inc., Class A 1.0%
Par Pacific Holdings, Inc. 0.9%
CONMED Corp. 0.9%
Dorian LPG Ltd. 0.8%
DXC Technology Co. 0.8%
International Seaways, Inc. 0.8%
PVH Corp. 0.7%
Adient PLC 0.7%
Sector Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
CBIZ, Inc. 1.1%
Everforth, Inc. 1.0%
PBF Energy, Inc., Class A 1.0%
Par Pacific Holdings, Inc. 0.9%
CONMED Corp. 0.9%
Dorian LPG Ltd. 0.8%
DXC Technology Co. 0.8%
International Seaways, Inc. 0.8%
PVH Corp. 0.7%
Adient PLC 0.7%
[1]
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
[2]
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
[3] Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
[4]
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
[5]
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
[6]
Excludes any Acquired Fund Fees and Expenses of the underlying investment companies in which the Fund invests.
[7]
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.
[8]
Includes extraordinary expenses. If these extraordinary expenses were not included, the expense ratio would have been 0.70%.