Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SUMMER STREET TRUST
NF_04.30 Fidelity SAI High Income Fund PRO-01 | Fidelity SAI High Income Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary Fund:Fidelity® SAI High Income Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks a high level of current income. Growth of capital may also be considered.
Expense [Heading]	rr_ExpenseHeading	Fee Table
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees
(fees paid directly from your investment)	rr_ShareholderFeeOther	none
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Operating Expenses (expenses that you pay each year as a % of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	Based on estimated amounts for the current fiscal year.
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Normally investing primarily in income producing debt securities, preferred stocks, and convertible securities, with an emphasis on lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds). Potentially investing in non-income producing securities, including defaulted securities and common stocks. Investing in companies in troubled or uncertain financial condition. Using the credit quality distribution of an index representing the overall high yield bond market, currently the ICE® BofA® US High Yield Constrained Index, as a guide in structuring the fund's credit quality composition. Investing in domestic and foreign issuers. Using fundamental analysis of each issuer's financial condition and industry position and market and economic conditions to select investments.
Risk [Heading]	rr_RiskHeading	Principal Investment Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments. Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease. Foreign Exposure. Foreign markets, particularly emerging markets, can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market. Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. Lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds) and certain types of other securities involve greater risk of default or price changes due to changes in the credit quality of the issuer. The value of lower-quality debt securities and certain types of other securities can be more volatile due to increased sensitivity to adverse issuer, political, regulatory, market, or economic developments and can be difficult to resell. An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You could lose money by investing in the fund.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Performance history will be available for the fund after the fund has been in operation for one calendar year.
NF_04.30 Fidelity SAI High Income Fund PRO-01 | Fidelity SAI High Income Fund | Fidelity SAI High Income Fund
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.02%	[1]
Total annual operating expenses	rr_ExpensesOverAssets	0.57%
1 year	rr_ExpenseExampleYear01	$ 58
3 years	rr_ExpenseExampleYear03	$ 183

[1] ^(a)Based on estimated amounts for the current fiscal year.