Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SUMMER STREET TRUST
05.31 Megatrend Funds Combo Pro-03
Risk/Return:	rr_RiskReturnAbstract
Supplement to Prospectus [Text Block]	rr_SupplementToProspectusTextBlock	Supplement to theFidelity® Agricultural Productivity Fund and Fidelity® Water Sustainability FundJuly 30, 2021Prospectus
05.31 Megatrend Funds Combo Pro-03 | Fidelity Water Sustainability Fund
Risk/Return:	rr_RiskReturnAbstract
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Employing sustainable investing exclusion criteria to avoid investments in issuers that are directly engaged in, and/or derive significant revenue from, certain industries. Please see “Fund Basics – Investment Details – Sustainable Investing Exclusions” for additional information.
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Sustainability Risk. Application of FMR's ESG ratings process and/or its sustainable investing exclusion criteria may affect the fund’s exposure to certain issuers, sectors, regions, and countries and may affect the fund’s performance depending on whether certain investments are in or out of favor. The criteria related to the fund’s ESG ratings process and/or adherence to its sustainable investing exclusion criteria may result in the fund forgoing opportunities to buy certain securities when it might otherwise be advantageous to do so, or selling securities for ESG reasons when it might be otherwise disadvantageous for it to do so. As a result, the fund’s performance may at times be better or worse than the performance of funds that do not use ESG or sustainability criteria. There are significant differences in interpretations of what it means for an issuer to have positive ESG factors. While the Adviser believes its definitions are reasonable, the portfolio decisions it makes may differ with other investors’ or advisers’ views.