v3.26.1
Shareholder Report
 6 Months Ended
Jun. 30, 2026
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Advisor Series I
Entity Central Index Key 0000722574
Entity Investment Company Type N-1A
Document Period End Date Jun. 30, 2026
Fidelity Advisor Mid Cap II Fund - Class Z  
Shareholder Report [Line Items]  
Fund Name Fidelity Advisor® Mid Cap II Fund
Class Name Fidelity Advisor® Mid Cap II Fund Class Z
Trading Symbol FZAMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Advisor® Mid Cap II Fund for the period January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class Z 
$ 32 
0.57%
  
Expenses Paid, Amount $ 32
Expense Ratio, Percent 0.57%
Net Assets $ 2,363,373,359
Holdings Count | shares 183
Investment Company Portfolio Turnover 78.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2026)
 
KEY FACTS 
  
  
Fund Size
$2,363,373,359
  
Number of Holdings
183
  
Portfolio Turnover
78%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Industrials 21.8   Information Technology 19.6   Financials 14.3   Health Care 9.6   Consumer Discretionary 9.2   Real Estate 5.3   Energy 4.6   Consumer Staples 4.4   Materials 3.4   Utilities 3.4   Communication Services 1.2     Common Stocks 96.7 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 3.2 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.7                       Preferred Stocks - 0.1                     Short-Term Investments and Net Other Assets (Liabilities) - 3.2   United States 93.8 Canada 2.0 Taiwan 1.4 Belgium 0.6 Japan 0.6 Brazil 0.6 United Kingdom 0.4 France 0.3 Netherlands 0.2 Others 0.1 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 93.8                       Canada - 2.0                               Taiwan - 1.4                               Belgium - 0.6                              Japan - 0.6                                Brazil - 0.6                               United Kingdom - 0.4                       France - 0.3                               Netherlands - 0.2                          Others - 0.1                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     MKS Inc 1.8   Twilio Inc Class A 1.6   East West Bancorp Inc 1.5   Silicon Motion Technology Corp ADR 1.4   US Foods Holding Corp 1.4   ITT Inc 1.4   Comfort Systems USA Inc 1.3   ATI Inc 1.3   Performance Food Group Co 1.2   RB Global Inc (United States) 1.2     14.1  
Fidelity Advisor Mid Cap II Fund - Class M  
Shareholder Report [Line Items]  
Fund Name Fidelity Advisor® Mid Cap II Fund
Class Name Fidelity Advisor® Mid Cap II Fund Class M
Trading Symbol FITIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Advisor® Mid Cap II Fund for the period January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class M 
$ 67 
1.19%
  
Expenses Paid, Amount $ 67
Expense Ratio, Percent 1.19%
Net Assets $ 2,363,373,359
Holdings Count | shares 183
Investment Company Portfolio Turnover 78.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2026)
 
KEY FACTS 
  
  
Fund Size
$2,363,373,359
  
Number of Holdings
183
  
Portfolio Turnover
78%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Industrials 21.8   Information Technology 19.6   Financials 14.3   Health Care 9.6   Consumer Discretionary 9.2   Real Estate 5.3   Energy 4.6   Consumer Staples 4.4   Materials 3.4   Utilities 3.4   Communication Services 1.2     Common Stocks 96.7 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 3.2 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.7                       Preferred Stocks - 0.1                     Short-Term Investments and Net Other Assets (Liabilities) - 3.2   United States 93.8 Canada 2.0 Taiwan 1.4 Belgium 0.6 Japan 0.6 Brazil 0.6 United Kingdom 0.4 France 0.3 Netherlands 0.2 Others 0.1 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 93.8                       Canada - 2.0                               Taiwan - 1.4                               Belgium - 0.6                              Japan - 0.6                                Brazil - 0.6                               United Kingdom - 0.4                       France - 0.3                               Netherlands - 0.2                          Others - 0.1                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     MKS Inc 1.8   Twilio Inc Class A 1.6   East West Bancorp Inc 1.5   Silicon Motion Technology Corp ADR 1.4   US Foods Holding Corp 1.4   ITT Inc 1.4   Comfort Systems USA Inc 1.3   ATI Inc 1.3   Performance Food Group Co 1.2   RB Global Inc (United States) 1.2     14.1  
Fidelity Advisor Mid Cap II Fund - Class I  
Shareholder Report [Line Items]  
Fund Name Fidelity Advisor® Mid Cap II Fund
Class Name Fidelity Advisor® Mid Cap II Fund Class I
Trading Symbol FIIMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Advisor® Mid Cap II Fund for the period January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class I 
$ 39 
0.69%
  
Expenses Paid, Amount $ 39
Expense Ratio, Percent 0.69%
Net Assets $ 2,363,373,359
Holdings Count | shares 183
Investment Company Portfolio Turnover 78.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2026)
 
KEY FACTS 
  
  
Fund Size
$2,363,373,359
  
Number of Holdings
183
  
Portfolio Turnover
78%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Industrials 21.8   Information Technology 19.6   Financials 14.3   Health Care 9.6   Consumer Discretionary 9.2   Real Estate 5.3   Energy 4.6   Consumer Staples 4.4   Materials 3.4   Utilities 3.4   Communication Services 1.2     Common Stocks 96.7 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 3.2 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.7                       Preferred Stocks - 0.1                     Short-Term Investments and Net Other Assets (Liabilities) - 3.2   United States 93.8 Canada 2.0 Taiwan 1.4 Belgium 0.6 Japan 0.6 Brazil 0.6 United Kingdom 0.4 France 0.3 Netherlands 0.2 Others 0.1 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 93.8                       Canada - 2.0                               Taiwan - 1.4                               Belgium - 0.6                              Japan - 0.6                                Brazil - 0.6                               United Kingdom - 0.4                       France - 0.3                               Netherlands - 0.2                          Others - 0.1                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     MKS Inc 1.8   Twilio Inc Class A 1.6   East West Bancorp Inc 1.5   Silicon Motion Technology Corp ADR 1.4   US Foods Holding Corp 1.4   ITT Inc 1.4   Comfort Systems USA Inc 1.3   ATI Inc 1.3   Performance Food Group Co 1.2   RB Global Inc (United States) 1.2     14.1  
Fidelity Advisor Mid Cap II Fund - Class C  
Shareholder Report [Line Items]  
Fund Name Fidelity Advisor® Mid Cap II Fund
Class Name Fidelity Advisor® Mid Cap II Fund Class C
Trading Symbol FIICX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Advisor® Mid Cap II Fund for the period January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class C 
$ 96 
1.69%
  
Expenses Paid, Amount $ 96
Expense Ratio, Percent 1.69%
Net Assets $ 2,363,373,359
Holdings Count | shares 183
Investment Company Portfolio Turnover 78.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2026)
 
KEY FACTS 
  
  
Fund Size
$2,363,373,359
  
Number of Holdings
183
  
Portfolio Turnover
78%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Industrials 21.8   Information Technology 19.6   Financials 14.3   Health Care 9.6   Consumer Discretionary 9.2   Real Estate 5.3   Energy 4.6   Consumer Staples 4.4   Materials 3.4   Utilities 3.4   Communication Services 1.2     Common Stocks 96.7 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 3.2 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.7                       Preferred Stocks - 0.1                     Short-Term Investments and Net Other Assets (Liabilities) - 3.2   United States 93.8 Canada 2.0 Taiwan 1.4 Belgium 0.6 Japan 0.6 Brazil 0.6 United Kingdom 0.4 France 0.3 Netherlands 0.2 Others 0.1 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 93.8                       Canada - 2.0                               Taiwan - 1.4                               Belgium - 0.6                              Japan - 0.6                                Brazil - 0.6                               United Kingdom - 0.4                       France - 0.3                               Netherlands - 0.2                          Others - 0.1                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     MKS Inc 1.8   Twilio Inc Class A 1.6   East West Bancorp Inc 1.5   Silicon Motion Technology Corp ADR 1.4   US Foods Holding Corp 1.4   ITT Inc 1.4   Comfort Systems USA Inc 1.3   ATI Inc 1.3   Performance Food Group Co 1.2   RB Global Inc (United States) 1.2     14.1  
Fidelity Advisor Mid Cap II Fund - Class A  
Shareholder Report [Line Items]  
Fund Name Fidelity Advisor® Mid Cap II Fund
Class Name Fidelity Advisor® Mid Cap II Fund Class A
Trading Symbol FIIAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Advisor® Mid Cap II Fund for the period January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class A 
$ 53 
0.94%
  
Expenses Paid, Amount $ 53
Expense Ratio, Percent 0.94%
Net Assets $ 2,363,373,359
Holdings Count | shares 183
Investment Company Portfolio Turnover 78.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2026)
 
KEY FACTS 
  
  
Fund Size
$2,363,373,359
  
Number of Holdings
183
  
Portfolio Turnover
78%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Industrials 21.8   Information Technology 19.6   Financials 14.3   Health Care 9.6   Consumer Discretionary 9.2   Real Estate 5.3   Energy 4.6   Consumer Staples 4.4   Materials 3.4   Utilities 3.4   Communication Services 1.2     Common Stocks 96.7 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 3.2 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.7                       Preferred Stocks - 0.1                     Short-Term Investments and Net Other Assets (Liabilities) - 3.2   United States 93.8 Canada 2.0 Taiwan 1.4 Belgium 0.6 Japan 0.6 Brazil 0.6 United Kingdom 0.4 France 0.3 Netherlands 0.2 Others 0.1 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 93.8                       Canada - 2.0                               Taiwan - 1.4                               Belgium - 0.6                              Japan - 0.6                                Brazil - 0.6                               United Kingdom - 0.4                       France - 0.3                               Netherlands - 0.2                          Others - 0.1                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     MKS Inc 1.8   Twilio Inc Class A 1.6   East West Bancorp Inc 1.5   Silicon Motion Technology Corp ADR 1.4   US Foods Holding Corp 1.4   ITT Inc 1.4   Comfort Systems USA Inc 1.3   ATI Inc 1.3   Performance Food Group Co 1.2   RB Global Inc (United States) 1.2     14.1