v3.25.2
Shareholder Report, Average Annual Return (Details)
 12 Months Ended 60 Months Ended 120 Months Ended
Jul. 31, 2025
Jul. 31, 2025
Jul. 31, 2025
C000047995 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Large Cap Core AlphaDEX® Fund    
Average Annual Return, Percent 13.12% 13.86% 10.75%
C000047996 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Mid Cap Core AlphaDEX® Fund    
Average Annual Return, Percent 2.23% 13.91% 9.49%
C000047997 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Small Cap Core AlphaDEX® Fund    
Average Annual Return, Percent (0.44%) 13.22% 8.48%
C000047998 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Large Cap Value AlphaDEX® Fund    
Average Annual Return, Percent 3.88% 13.51% 8.76%
C000047999 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Large Cap Growth AlphaDEX® Fund    
Average Annual Return, Percent 26.96% 14.19% 12.66%
C000048000 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Multi Cap Value AlphaDEX® Fund    
Average Annual Return, Percent (1.53%) 14.02% 8.16%
C000048001 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Multi Cap Growth AlphaDEX® Fund    
Average Annual Return, Percent 19.06% 13.03% 11.62%
C000048896 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Consumer Discretionary AlphaDEX® Fund    
Average Annual Return, Percent 7.69% 11.13% 6.61%
C000048897 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Consumer Staples AlphaDEX® Fund    
Average Annual Return, Percent (2.69%) 7.08% 5.04%
C000048898 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Energy AlphaDEX® Fund    
Average Annual Return, Percent (11.32%) 23.01% 1.59%
C000048899 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Financials AlphaDEX® Fund    
Average Annual Return, Percent 15.31% 20.09% 11.34%
C000048900 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Health Care AlphaDEX® Fund    
Average Annual Return, Percent (9.12%) 0.94% 3.67%
C000048901 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Industrials/Producer Durables AlphaDEX® Fund    
Average Annual Return, Percent 6.30% 15.45% 11.05%
C000048902 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Materials AlphaDEX® Fund    
Average Annual Return, Percent (12.88%) 11.77% 8.14%
C000048903 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Technology AlphaDEX® Fund    
Average Annual Return, Percent 18.71% 12.59% 16.86%
C000048904 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Utilities AlphaDEX® Fund    
Average Annual Return, Percent 30.64% 12.74% 9.77%
C000099059 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Mid Cap Growth AlphaDEX® Fund    
Average Annual Return, Percent 10.64% 10.77% 10.71%
C000099060 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Mid Cap Value AlphaDEX® Fund    
Average Annual Return, Percent (4.02%) 15.57% 7.68%
C000099061 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Small Cap Growth AlphaDEX® Fund    
Average Annual Return, Percent 12.93% 12.91% 9.87%
C000099062 [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name First Trust Small Cap Value AlphaDEX® Fund    
Average Annual Return, Percent (10.75%) 12.41% 6.92%
Standard & Poor's 500 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Index    
Average Annual Return, Percent 16.33% 15.88% 13.66%
Russell 3000 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 3000® Index    
Average Annual Return, Percent 15.68% 15.19% 13.02%
S And P Composite 1500 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P Composite 1500® Index    
Average Annual Return, Percent 15.03% 15.60% 13.27%
Russell 1000 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Index    
Average Annual Return, Percent 16.54% 15.49% 13.38%
Nasdaq AlphaDEX Large Cap CoreTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Large Cap CoreTMIndex    
Average Annual Return, Percent [1] 13.79% 14.56%
Nasdaq US 500 Large CapTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 500 Large CapTMIndex    
Average Annual Return, Percent [1] 17.67% 15.72%
S And P MidCap 400 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P MidCap 400® Index    
Average Annual Return, Percent 3.28% 12.79% 9.41%
Nasdaq AlphaDEX Mid Cap CoreTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Mid Cap CoreTMIndex    
Average Annual Return, Percent [2] 2.83% 14.62%
Nasdaq US 600 Mid CapTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 600 Mid CapTMIndex    
Average Annual Return, Percent [2] 4.95% 11.11%
Nasdaq AlphaDEX Small Cap CoreTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Small Cap CoreTMIndex    
Average Annual Return, Percent [3] 0.22% 13.99%
Nasdaq US 700 Small CapTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 700 Small CapTMIndex    
Average Annual Return, Percent [3] 2.38% 11.51%
S And P SmallCap 600 Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P SmallCap 600® Index    
Average Annual Return, Percent (4.72%) 10.99% 8.21%
Nasdaq AlphaDEX Large Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Large Cap ValueTMIndex    
Average Annual Return, Percent [4] 4.47% 14.22%
Nasdaq US 500 Large Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 500 Large Cap ValueTMIndex    
Average Annual Return, Percent [4] 2.60% 13.43%
S And P 500 Value Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Value Index    
Average Annual Return, Percent 5.59% 14.40% 10.47%
Nasdaq US 500 Large Cap Growth [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 500 Large Cap GrowthTMIndex    
Average Annual Return, Percent [5] 24.68% 16.42%
Nasdaq AlphaDEX Large Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Large Cap GrowthTMIndex    
Average Annual Return, Percent [5] 27.73% 14.87%
S And P 500 Growth Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Growth Index    
Average Annual Return, Percent 25.63% 16.50% 15.93%
Nasdaq AlphaDEX Multi Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Multi Cap ValueTMIndex    
Average Annual Return, Percent [6] (0.87%) 14.81%
Nasdaq US Multi Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US Multi Cap ValueTMIndex    
Average Annual Return, Percent [6] 1.97% 13.54%
S And P Composite 1500 Value Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P Composite 1500® Value Index    
Average Annual Return, Percent 5.25% 14.43% 10.33%
Nasdaq AlphaDEX Multi Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Multi Cap GrowthTMIndex    
Average Annual Return, Percent [7] 19.85% 13.78%
Nasdaq US Multi Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US Multi Cap GrowthTMIndex    
Average Annual Return, Percent [7] 23.70% 15.74%
S And P Composite 1500 Growth Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P Composite 1500® Growth Index    
Average Annual Return, Percent 23.48% 16.02% 15.35%
Russell 1000 Consumer Discretionary Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Consumer Discretionary Index    
Average Annual Return, Percent 22.68% 11.78% 12.73%
StrataQuant Consumer Discretionary Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Consumer Discretionary Index    
Average Annual Return, Percent 8.26% 11.80% 7.26%
StrataQuant Consumer Staples Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Consumer Staples Index    
Average Annual Return, Percent (2.08%) 7.69% 5.69%
Russell 1000 Consumer Staples Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Consumer Staples Index    
Average Annual Return, Percent 3.68% 7.62% 6.53%
S And P 500 Consumer Staples Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Consumer Staples Index    
Average Annual Return, Percent 7.43% 9.19% 8.35%
S And P 500 Consumer Discretionary Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Consumer Discretionary Index    
Average Annual Return, Percent 19.54% 10.32% 12.19%
StrataQuant Energy Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Energy Index    
Average Annual Return, Percent (10.79%) 23.91% 2.21%
S And P 500 Energy Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Energy Index    
Average Annual Return, Percent (3.19%) 24.55% 6.64%
Russell 1000 Energy Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Energy Index    
Average Annual Return, Percent (2.40%) 24.10% 6.10%
StrataQuant Financials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Financials Index    
Average Annual Return, Percent 16.02% 20.86% 12.08%
S And P 500 Financials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Financials Index    
Average Annual Return, Percent 21.54% 19.09% 12.11%
Russell 1000 Financials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Financials Index    
Average Annual Return, Percent 22.47% 20.20% 13.89%
StrataQuant Health Care Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Health Care Index    
Average Annual Return, Percent (8.68%) 1.55% 4.33%
S And P 500 Health Care Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Health Care Index    
Average Annual Return, Percent (11.32%) 6.09% 7.38%
Russell 1000 Health Care Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Health Care Index    
Average Annual Return, Percent (11.23%) 5.40% 7.22%
StrataQuant Industrials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Industrials Index    
Average Annual Return, Percent 6.98% 16.23% 11.81%
S And P 500 Industrials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Industrials Index    
Average Annual Return, Percent 20.67% 18.13% 12.74%
Russell 1000 Industrials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Industrials Index    
Average Annual Return, Percent 15.29% 14.51% 11.31%
StrataQuant Materials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Materials Index    
Average Annual Return, Percent (12.24%) 12.52% 8.89%
S And P 500 Materials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Materials Index    
Average Annual Return, Percent (2.85%) 9.94% 8.96%
Russell 1000 Basic Materials Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Basic Materials Index    
Average Annual Return, Percent 0.27% 10.44% 9.41%
StrataQuant Technology Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant Technology® Index    
Average Annual Return, Percent 19.52% 13.23% 17.51%
S And P 500 Information Technology Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Information Technology Index    
Average Annual Return, Percent 23.65% 22.92% 23.47%
Russell 1000 Technology Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Technology Index    
Average Annual Return, Percent 26.14% 22.86% 23.19%
StrataQuant Utilities Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name StrataQuant® Utilities Index    
Average Annual Return, Percent 31.67% 13.55% 10.51%
S And P 500 Utilities Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P 500® Utilities Index    
Average Annual Return, Percent 21.26% 10.54% 10.54%
Russell 1000 Utilities Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Russell 1000® Utilities Index    
Average Annual Return, Percent 21.02% 11.69% 10.12%
Nasdaq AlphaDEX Mid Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Mid Cap GrowthTMIndex    
Average Annual Return, Percent [8] 11.48% 11.59%
Nasdaq US 600 Mid Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 600 Mid Cap GrowthTMIndex    
Average Annual Return, Percent [8] 10.40% 7.69%
S And P MidCap 400 Growth Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P MidCap 400® Growth Index    
Average Annual Return, Percent 2.24% 10.46% 9.20%
S And P MidCap 400 Value Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P MidCap 400® Value Index    
Average Annual Return, Percent 4.32% 15.11% 9.30%
Nasdaq AlphaDEX Mid Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Mid Cap ValueTMIndex    
Average Annual Return, Percent [9] (3.32%) 16.46%
Nasdaq US 600 Mid Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 600 Mid Cap ValueTMIndex    
Average Annual Return, Percent [9] (0.54%) 14.07%
Nasdaq AlphaDEX Small Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Small Cap GrowthTMIndex    
Average Annual Return, Percent [10] 13.80% 13.79%
Nasdaq US 700 Small Cap GrowthTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 700 Small Cap GrowthTMIndex    
Average Annual Return, Percent [10] 10.53% 9.60%
S And P SmallCap 600 Growth Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P SmallCap 600® Growth Index    
Average Annual Return, Percent (4.14%) 9.58% 8.56%
S And P SmallCap 600 Value Index [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name S&P SmallCap 600® Value Index    
Average Annual Return, Percent (5.45%) 12.28% 7.68%
Nasdaq US 700 Small Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq US 700 Small Cap ValueTMIndex    
Average Annual Return, Percent [11] (5.97%) 13.07%
Nasdaq AlphaDEX Small Cap ValueTMIndex [Member]      
Average Annual Return [Line Items]      
Line Graph and Table Measure Name Nasdaq AlphaDEX Small Cap ValueTMIndex    
Average Annual Return, Percent [11] (10.04%) 13.31%
[1] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[2] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[3] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[4] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[5] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[6] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[7] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[8] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[9] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[10] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.
[11] Performance data is not available for all the periods shown in the table for the index because performance data does not exist for some of the entire periods.