Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY INCOME FUND /MA/
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary Fund/Class:Fidelity® Environmental Bond Fund/Fidelity Advisor® Environmental Bond Fund A, M, C, I, Z
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks a high level of current income.
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. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 33 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees (fees paid directly from your investment)
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. During the most recent fiscal year, the fund's portfolio turnover rate was 198% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	198.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 33 of the prospectus.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
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 and if you hold your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Normally investing at least 80% of assets in environmental debt securities of all types and repurchase agreements for those securities. Environmental debt securities include debt securities of issuers that develop or provide products or services that seek to provide environmental solutions and/or support efforts to reduce their own environmental footprint, and debt securities that support environmental projects. Environmental debt securities include issuers, including sovereign or government-related issuers, (i) contained in the Bloomberg MSCI Global Green Bond USD Index (Hedged USD) and/or (ii) that have an above average environmental score as categorized by MSCI or are rated low risk or better by Sustainalytics. Normally investing primarily in investment-grade debt securities. Managing the fund with reference to the overall interest rate risk of the Bloomberg U.S. Aggregate Bond Index. Investing in domestic and foreign issuers. Allocating assets across different market sectors and maturities. Investing up to 20% of assets in lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds). Hedging the fund's foreign currency exposures utilizing forward foreign currency exchange contracts. Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments. Engaging in transactions that have a leveraging effect on the fund, including investments in derivatives - such as swaps (interest rate, total return, and credit default), options, and futures contracts - and forward-settling securities, to adjust the fund's risk exposure. 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 [Heading]	rr_RiskHeading	Principal Investment Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Environmental Focus Risk. The fund normally invests in issuers whose products or services are intended or likely to have a positive environmental impact. Some industries and sectors might be more likely to issue environmental bonds, and events or factors impacting these sectors may have a greater effect on, and may more adversely affect, the fund than they would a fund that does not invest in issuers with a common purpose. The fund also will be affected by a decrease in governmental or other support for environmental initiatives. 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. When evaluating an issuer, the Adviser is dependent on information or data obtained through voluntary or third-party reporting that may be incomplete, inaccurate, or unavailable, which could cause the Adviser to incorrectly assess an issuer’s business practices. 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. The extent of economic development; political stability; market depth, infrastructure, and capitalization; and regulatory oversight can be less than in more developed markets. Emerging markets typically have less established legal, accounting and financial reporting systems than those in more developed markets, which may reduce the scope or quality of financial information available to investors. Emerging markets can be subject to greater social, economic, regulatory, and political uncertainties and can be extremely volatile. Foreign Currency Transactions. Although a forward foreign currency exchange contract is used to reduce or hedge a fund’s exposure to changes in the value of the currency, suitable hedging transactions may not be available in all circumstances, may not be successful, and may eliminate any chance for the fund to benefit from favorable fluctuations in relevant foreign currencies. Prepayment. The ability of an issuer of a debt security to repay principal prior to a security's maturity can cause greater price volatility if interest rates change. 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease. Lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds) 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 can be more volatile due to increased sensitivity to adverse issuer, political, regulatory, market, or economic developments. Leverage Risk. Leverage can increase market exposure, magnify investment risks, and cause losses to be realized more quickly. High Portfolio Turnover. High portfolio turnover (more than 100%) may result in increased transaction costs and potentially higher capital gains or losses. The effects of higher than normal portfolio turnover may adversely affect the fund's performance. 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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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.
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund | Fidelity Environmental Bond Fund-Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) on purchases (as a % of offering price)	rr_MaximumCumulativeSalesChargeOverOfferingPrice	4.00%
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOther	none	[1]
Management fee	rr_ManagementFeesOverAssets	0.35%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.07%
Total annual operating expenses	rr_ExpensesOverAssets	0.67%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.67%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	Class A and Class M purchases of $1 million or more will not be subject to a front-end sales charge. Such Class A and Class M purchases may be subject, upon redemption, to a contingent deferred sales charge (CDSC) of 0.75% or 0.25%, respectively.
1 year	rr_ExpenseExampleYear01	$ 466
3 years	rr_ExpenseExampleYear03	606
5 years	rr_ExpenseExampleYear05	758
10 years	rr_ExpenseExampleYear10	1,201
1 Year	rr_ExpenseExampleNoRedemptionYear01	466
3 Years	rr_ExpenseExampleNoRedemptionYear03	606
5 Years	rr_ExpenseExampleNoRedemptionYear05	758
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,201
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund | Fidelity Environmental Bond Fund-Class M
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) on purchases (as a % of offering price)	rr_MaximumCumulativeSalesChargeOverOfferingPrice	4.00%
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOther	none	[1]
Management fee	rr_ManagementFeesOverAssets	0.35%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.07%
Total annual operating expenses	rr_ExpensesOverAssets	0.67%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.67%
1 year	rr_ExpenseExampleYear01	$ 466
3 years	rr_ExpenseExampleYear03	606
5 years	rr_ExpenseExampleYear05	758
10 years	rr_ExpenseExampleYear10	1,201
1 Year	rr_ExpenseExampleNoRedemptionYear01	466
3 Years	rr_ExpenseExampleNoRedemptionYear03	606
5 Years	rr_ExpenseExampleNoRedemptionYear05	758
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,201
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund | Fidelity Environmental Bond Fund-Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) on purchases (as a % of offering price)	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[2]
Management fee	rr_ManagementFeesOverAssets	0.35%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.07%
Total annual operating expenses	rr_ExpensesOverAssets	1.42%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	1.42%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	On Class C shares redeemed less than one year after purchase.
1 year	rr_ExpenseExampleYear01	$ 245
3 years	rr_ExpenseExampleYear03	449
5 years	rr_ExpenseExampleYear05	776
10 years	rr_ExpenseExampleYear10	1,497
1 Year	rr_ExpenseExampleNoRedemptionYear01	145
3 Years	rr_ExpenseExampleNoRedemptionYear03	449
5 Years	rr_ExpenseExampleNoRedemptionYear05	776
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,497
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund | Fidelity Environmental Bond Fund-Class I Advisor
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) on purchases (as a % of offering price)	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOther	none
Management fee	rr_ManagementFeesOverAssets	0.35%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.25%
Total annual operating expenses	rr_ExpensesOverAssets	0.60%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.60%
1 year	rr_ExpenseExampleYear01	$ 61
3 years	rr_ExpenseExampleYear03	192
5 years	rr_ExpenseExampleYear05	335
10 years	rr_ExpenseExampleYear10	750
1 Year	rr_ExpenseExampleNoRedemptionYear01	61
3 Years	rr_ExpenseExampleNoRedemptionYear03	192
5 Years	rr_ExpenseExampleNoRedemptionYear05	335
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 750
08.31 Fidelity Environmental Bond Fund AMCIZ Pro-04 | Fidelity Environmental Bond Fund | Fidelity Environmental Bond Fund-Class Z
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) on purchases (as a % of offering price)	rr_MaximumCumulativeSalesChargeOverOfferingPrice	none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOther	none
Management fee	rr_ManagementFeesOverAssets	0.35%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.05%
Total annual operating expenses	rr_ExpensesOverAssets	0.40%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.04%	[3]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.36%
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2023
1 year	rr_ExpenseExampleYear01	$ 37
3 years	rr_ExpenseExampleYear03	123
5 years	rr_ExpenseExampleYear05	219
10 years	rr_ExpenseExampleYear10	500
1 Year	rr_ExpenseExampleNoRedemptionYear01	37
3 Years	rr_ExpenseExampleNoRedemptionYear03	123
5 Years	rr_ExpenseExampleNoRedemptionYear05	219
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 500

[1] ^(a)Class A and Class M purchases of $1 million or more will not be subject to a front-end sales charge. Such Class A and Class M purchases may be subject, upon redemption, to a contingent deferred sales charge (CDSC) of 0.75% or 0.25%, respectively.

[2] ^(b)On Class C shares redeemed less than one year after purchase.

[3] ^(a)Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class Z of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.36% (the Expense Cap). If at any time during the current fiscal year expenses for Class Z of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through December 31, 2023 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.