| Form N-CSR Cover | 6 Months Ended |
| Jul. 31, 2024 | |
| 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, 2024 |
| Shareholder Report | 6 Months Ended |
| Jul. 31, 2024 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, 2024 |
| 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, 2024 to July 31, 2024. |
| 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 $ A 0.00 % B A Amount represents less than $.50 B Amount represents less than 0.005% |
| Expenses Paid, Amount | $ 0 |
| Expense Ratio, Percent | 0% |
| Net Assets | $ 21,662,749 |
| Holdings Count | shares | 776 |
| Investment Company Portfolio Turnover | 22% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of July 31, 2024) KEY FACTS Fund Size $ Number of Holdings 776 Portfolio Turnover 22 % |
| Holdings [Text Block] | MARKET SECTORS (% of Fund's net assets) Information Technology 27.4 Financials 12.9 Health Care 12.1 Industrials 10.4 Consumer Discretionary 9.0 Communication Services 6.9 Consumer Staples 5.5 Energy 5.2 Materials 2.9 Utilities 1.9 Real Estate 1.7 Common Stocks 95.8 Preferred Stocks 0.1 Short-Term Investments and Net Other Assets (Liabilities) 4.1 ASSET ALLOCATION (% of Fund's net assets) United States 94.9 Canada 1.0 United Kingdom 0.6 China 0.5 Germany 0.4 France 0.3 Taiwan 0.3 Ireland 0.3 India 0.2 Others 1.5 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets) |
| Largest Holdings [Text Block] | TOP HOLDINGS (% of Fund's net assets) NVIDIA Corp 5.6 Apple Inc 4.8 Microsoft Corp 4.8 Amazon.com Inc 3.0 Alphabet Inc Class A 1.9 Alphabet Inc Class C 1.6 UnitedHealth Group Inc 1.6 JPMorgan Chase & Co 1.5 General Electric Co 1.3 Eli Lilly & Co 1.3 27.4 |
| Shareholder Report, Holdings (Details) - Fidelity Series Sustainable U.S. Market Fund | Jul. 31, 2024 |
| TH0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.60% |
| TH1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| TH2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| TH3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3% |
| TH4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| TH5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| TH6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| TH7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| TH8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| TH9Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| MSFinancialsSectorMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.90% |
| Information Technology Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 27.40% |
| Healthcare Sector | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.10% |
| Real Estate Sector | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| Industrial Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.40% |
| Materials Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| Consumer Discretionary Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9% |
| Consumer Staples Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.50% |
| Communications Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.90% |
| Energy Sector | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.20% |
| Utilities Sector Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| United States | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 94.90% |
| Canada | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| United Kingdom | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.60% |
| China | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| Germany | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| France | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| Taiwan | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| Ireland | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| India | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| OtherMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| AACommonStocksMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 95.80% |
| AAPreferredStocksMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.10% |
| AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.10% |
