v3.26.3
Shareholder Report
 6 Months Ended
Jul. 31, 2026
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Summer Street Trust
Entity Central Index Key 0000225322
Entity Investment Company Type N-1A
Document Period End Date Jul. 31, 2026
Fidelity Series Sustainable U.S. Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Series Sustainable U.S. Market Fund
Class Name Fidelity® Series Sustainable U.S. Market Fund
Trading Symbol FSUMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Series Sustainable U.S. Market Fund for the period February 1, 2026 to July 31, 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 1-800-544-8544.
Additional Information Phone Number 1-800-544-8544
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
 
Fidelity® Series Sustainable U.S. Market Fund 
$ 0 A
0.00%B
  
AAmount represents less than $.50
BAmount represents less than 0.005%
 
Expenses Paid, Amount $ 0
Expense Ratio, Percent 0.00%
Net Assets $ 70,141,697
Holdings Count | shares 723
Investment Company Portfolio Turnover 35.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$70,141,697
  
Number of Holdings
723
  
Portfolio Turnover
35%
  
Holdings [Text Block]       MARKET SECTORS (% of Fund's net assets)     Information Technology 33.3   Financials 12.1   Consumer Discretionary 10.0   Health Care 10.0   Industrials 9.7   Communication Services 7.1   Energy 4.3   Consumer Staples 4.3   Materials 2.1   Real Estate 1.9   Utilities 1.4     Common Stocks 96.2 Preferred Stocks 0.0 Short-Term Investments and Net Other Assets (Liabilities) 3.8 ASSET ALLOCATION (% of Fund's net assets)     Common Stocks - 96.2                       Preferred Stocks - 0.0                     Short-Term Investments and Net Other Assets (Liabilities) - 3.8   United States 96.3 Canada 1.1 United Kingdom 0.8 Taiwan 0.5 Netherlands 0.3 Belgium 0.2 Puerto Rico 0.1 Switzerland 0.1 Japan 0.1 Others 0.5 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 96.3                       Canada - 1.1                               United Kingdom - 0.8                       Taiwan - 0.5                               Netherlands - 0.3                          Belgium - 0.2                              Puerto Rico - 0.1                          Switzerland - 0.1                          Japan - 0.1                                Others - 0.5                                  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     NVIDIA Corp 6.9   Apple Inc 6.0   Amazon.com Inc 4.9   Microsoft Corp 4.5   Alphabet Inc Class A 3.3   Broadcom Inc 2.2   Alphabet Inc Class C 1.8   ExxonMobil Holdings Corp 1.7   Eli Lilly & Co 1.2   Micron Technology Inc 1.1     33.6