Risk/Return Detail Data - FidelitySeriesSustainableUSMarketFund-PRO - USD ($) |
12 Months Ended | 36 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Apr. 01, 2026 |
Apr. 01, 2026 |
Dec. 31, 2025 |
Dec. 31, 2024 |
Apr. 01, 2026 |
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| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | Fidelity Series Sustainable U.S. Market Fund | ||||||||||
| Prospectus Line Items | ||||||||||
| Management Fees (as a percentage of Assets) | 0.00% | |||||||||
| Distribution and Service (12b-1) Fees | 0.00% | |||||||||
| Other Expenses (as a percentage of Assets): | 0.02% | |||||||||
| Expenses (as a percentage of Assets) | 0.02% | |||||||||
| Fee Waiver or Reimbursement | [1] | (0.02%) | ||||||||
| Net Expenses (as a percentage of Assets) | 0.00% | |||||||||
| Expense Example, with Redemption, 1 Year | $ 0 | |||||||||
| Expense Example, with Redemption, 3 Years | 0 | |||||||||
| Expense Example, with Redemption, 5 Years | 4 | |||||||||
| Expense Example, with Redemption, 10 Years | $ 19 | |||||||||
| Annual Return, Inception Date | May 11, 2023 | |||||||||
| Average Annual Return, Label [Optional Text] | Return Before Taxes | |||||||||
| Average Annual Return, Percent | 18.78% | 22.69% | [2] | |||||||
| Annual Return [Percent] | 18.78% | 23.10% | ||||||||
| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | ||||||||||
| Prospectus Line Items | ||||||||||
| Risk/Return [Heading] | Fund Summary Fund: Fidelity® Series Sustainable U.S. Market Fund | |||||||||
| Objective [Heading] | Investment Objective | |||||||||
| Objective, Primary [Text Block] | Fidelity® Series Sustainable U.S. Market Fund seeks long-term growth of capital. | |||||||||
| Expense Heading [Optional Text] | Fee Table | |||||||||
| Expense Narrative [Text Block] | The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. | |||||||||
| Shareholder Fees Caption [Optional Text] | Shareholder fees | |||||||||
| Shareholder Fee, Other | $ 0 | |||||||||
| Operating Expenses Caption [Optional Text] | Annual Operating Expenses | |||||||||
| Fee Waiver or Reimbursement over Assets, Date of Termination | May 31, 2029 | |||||||||
| Portfolio Turnover [Heading] | Portfolio Turnover | |||||||||
| Portfolio Turnover [Text Block] | 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. During the most recent fiscal year, the fund's portfolio turnover rate was 28 % of the average value of its portfolio. | |||||||||
| Portfolio Turnover, Rate | 28.00% | |||||||||
| Expense Example Narrative [Text Block] | 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 the 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] | Principal Investment Strategies | |||||||||
| Strategy Narrative [Text Block] | Normally investing at least 80% of assets in securities of issuers in the U.S. that Fidelity Management & Research Company LLC (FMR) (the Adviser) believes have proven or improving sustainability practices, based on an evaluation of such companies' individual environmental, social and governance (ESG) profile. The Adviser considers a number of factors to determine whether an issuer is located in or tied economically to a particular country or region including: whether a third-party vendor has assigned a particular country or region classification to the issuer or included the issuer in an index representative of a particular country or region; the issuer's domicile, incorporation, and location of assets; whether the issuer derives at least 50% of its revenues from, or has at least 50% of its assets in, a particular country or region; the source of government guarantees (if any); and the primary trading market or listing exchange. Whether an issuer is located in or tied economically to a particular country can be determined under any of these factors. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Normally investing primarily in equity securities. Using a quantitative portfolio construction and optimization model to select securities for the fund and capitalize on the Adviser's proprietary and third-party ESG ratings and research. Using the Adviser's proprietary ESG ratings process to evaluate the current state of an issuer's sustainability practices using a data-driven framework that includes both proprietary and third-party data, and also provide a qualitative forward-looking assessment of an issuer's sustainability outlook provided by the Adviser's fundamental research analysts and ESG team. The Adviser's ESG ratings of issuers are derived from multiple factors, including an issuer's environmental profile, which may include, but is not limited to, carbon and toxic emissions, water management, waste management, vulnerability to the physical impacts of climate change, and research and investment into products, services, and energies that reduce emissions and/or provide opportunities to transition to less carbon-intensive products or operations. An assessment of an issuer's social profile includes, but is not limited to, its approach to diversity and inclusion, human capital management, data privacy, product safety and human rights. With respect to governance, the independence and diversity of an issuer's board, its compensation practices and board oversight of critical ESG issues are considered as part of the assessment. These factors are weighted based on how material the Adviser believes each factor is to an issuer's financial outlook, and not all factors may be applicable to all issuers. Third-party ESG ratings will be considered in addition to the Adviser's ESG ratings in the quantitative portfolio construction and optimization process. Investing in either "growth" stocks or "value" stocks or both. Investing in domestic and foreign issuers. In addition to the ESG ratings process, using fundamental analysis of factors such as each issuer's financial condition and industry position, as well as market and economic conditions, to select investments. 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. | |||||||||
| Bar Chart and Performance Table [Heading] | Performance | |||||||||
| Performance Narrative [Text Block] | The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. The performance shown does not reflect the impact of any fees paid at the fee-based account or plan level, if applicable. Visit www.fidelity.com for more recent performance information. | |||||||||
| Performance Information Illustrates Variability of Returns [Text] | The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. | |||||||||
| Performance Availability Website Address [Text] | www.fidelity.com | |||||||||
| Performance Past Does Not Indicate Future [Text] | Past performance (before and after taxes) is not an indication of future performance. | |||||||||
| Bar Chart [Heading] | Year-by-Year Returns | |||||||||
| Highest Quarterly Return, Label [Optional Text] | Highest Quarter Return | |||||||||
| Highest Quarterly Return, Date | Jun. 30, 2025 | |||||||||
| Highest Quarterly Return | 11.43% | |||||||||
| Lowest Quarterly Return, Label [Optional Text] | Lowest Quarter Return | |||||||||
| Lowest Quarterly Return, Date | Mar. 31, 2025 | |||||||||
| Lowest Quarterly Return | (5.53%) | |||||||||
| Performance Table Heading | Average Annual Returns | |||||||||
| Performance Table Uses Highest Federal Rate | After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. | |||||||||
| Performance Table Not Relevant to Tax Deferred | The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). | |||||||||
| Performance Table Explanation after Tax Higher | Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares. | |||||||||
| Performance Table Narrative | After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares. | |||||||||
| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | After Taxes on Distributions | Fidelity Series Sustainable U.S. Market Fund | ||||||||||
| Prospectus Line Items | ||||||||||
| Average Annual Return, Label [Optional Text] | Return After Taxes on Distributions | |||||||||
| Average Annual Return, Percent | 18.41% | 22.16% | [2] | |||||||
| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | After Taxes on Distributions and Sales | Fidelity Series Sustainable U.S. Market Fund | ||||||||||
| Prospectus Line Items | ||||||||||
| Average Annual Return, Label [Optional Text] | Return After Taxes on Distributions and Sale of Fund Shares | |||||||||
| Average Annual Return, Percent | 11.32% | 17.80% | [2] | |||||||
| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | IXUIF | ||||||||||
| Prospectus Line Items | ||||||||||
| Average Annual Return, Label [Optional Text] | MSCI USA IMI ESG Focus Dynamic Weighted Index | |||||||||
| Average Annual Return, Percent | 16.47% | 21.63% | ||||||||
| FidelitySeriesSustainableUSMarketFund-PRO | Fidelity Series Sustainable U.S. Market Fund | WA006 | ||||||||||
| Prospectus Line Items | ||||||||||
| Average Annual Return, Label [Optional Text] | Dow Jones U.S. Total Stock Market Index℠ | |||||||||
| Average Annual Return, Percent | 17.05% | 22.28% | ||||||||
| Document Type | 485BPOS | |||||||||
| Registrant Name | Fidelity Summer Street Trust | |||||||||
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