| Label | Element | Value |
|---|---|---|
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Registrant Name | dei_EntityRegistrantName | Vanguard Fixed Income Securities Funds |
| Prospectus Date | rr_ProspectusDate | May 24, 2024 |
| Institutional Plus | Vanguard Real Estate II Index Fund | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk/Return [Heading] | rr_RiskReturnHeading | <span style="color:#000000;font-family:Arial;font-size:13pt;font-weight:bold;">Fund Summary</span> |
| Objective [Heading] | rr_ObjectiveHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Investment Objective</span> |
| Objective, Primary [Text Block] | rr_ObjectivePrimaryTextBlock | The Fund seeks to provide a high level of income and moderate long-term capital appreciation by tracking the performance of a benchmark index that measures the performance of publicly traded equity REITs and other real estate-related investments. |
| Expense [Heading] | rr_ExpenseHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Fees and Expenses</span> |
| Expense Narrative [Text Block] | rr_ExpenseNarrativeTextBlock | The following tables describe the fees and expenses you may pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and example below. |
| Shareholder Fees Caption [Text] | rr_ShareholderFeesCaption | <span style="color:#000000;font-family:Arial;font-size:8.5pt;font-weight:bold;">Shareholder Fees</span> <br/><span style="color:#000000;font-family:Arial;font-size:8.5pt;margin-left:0%;">(Fees paid directly from your investment)</span> |
| Operating Expenses Caption [Text] | rr_OperatingExpensesCaption | <span style="color:#000000;font-family:Arial;font-size:8.5pt;font-weight:bold;">Annual Fund Operating Expenses</span> <br/><span style="color:#000000;font-family:Arial;font-size:8.5pt;margin-left:0%;">(Expenses that you pay each year as a percentage of the value of your investment)</span> |
| Portfolio Turnover [Heading] | rr_PortfolioTurnoverHeading | <span style="color:#000000;font-family:Arial;font-size:9.5pt;">Portfolio Turnover</span> |
| 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 more taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the previous expense example, reduce the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 6% of the average value of its portfolio. |
| Portfolio Turnover, Rate | rr_PortfolioTurnoverRate | 6.00% |
| Expense Example [Heading] | rr_ExpenseExampleHeading | <span style="color:#000000;font-family:Arial;font-size:9.5pt;">Example</span> |
| Expense Example Narrative [Text Block] | rr_ExpenseExampleNarrativeTextBlock | The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. It illustrates the hypothetical expenses that you would incur over various periods if you were to invest $10,000 in the Fund’s shares. This example assumes that the Fund provides a return of 5% each year and that total annual fund operating expenses remain as stated in the preceding table. You would incur these hypothetical expenses whether or not you were to redeem your investment at the end of the given period. Although your actual costs may be higher or lower, based on these assumptions your costs would be: |
| Strategy [Heading] | rr_StrategyHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Principal Investment Strategies</span> |
| Strategy Narrative [Text Block] | rr_StrategyNarrativeTextBlock | The Fund employs an indexing investment approach designed to track the performance of the MSCI US Investable Market Real Estate 25/50 Index (the Index), an index that is made up of stocks of large, mid-size, and small U.S. companies within the real estate sector, as classified under the Global Industry Classification Standard (GICS). The GICS real estate sector is composed of equity real estate investment trusts (known as REITs), which include specialized REITs, and real estate management and development companies.The Fund attempts to track the Index by investing all, or substantially all, of its assets in the stocks that make up the Index, holding each stock in approximately the same proportion as its weighting in the Index. |
| Risk [Heading] | rr_RiskHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Principal Risks</span> |
| Risk [Text Block] | rr_RiskTextBlock | • Industry concentration risk, which is the chance that the stocks of REITs and other real estate-related investments will decline because of adverse developments affecting the real estate industry and real property values. Because the Fund concentrates its assets in these stocks, industry concentration risk is high.• Interest rate risk, which is the chance that REIT stock prices overall will decline and that the cost of borrowing for REITs will increase because of rising interest rates. Interest rate risk is high for the Fund.• Stock market risk, which is the chance that stock prices overall will decline. Stock markets tend to move in cycles, with periods of rising prices and periods of falling prices. The Fund’s target index may, at times, become focused in stocks of a limited number of companies, which could cause the Fund to underperform the overall stock market.• Investment style risk, which is the chance that returns from the stocks of REITs and other real estate-related investments—which typically are small- or mid-capitalization stocks—will trail returns from the overall stock market. Historically, these stocks have performed quite differently from the overall market.• Index replicating risk, which is the chance that the Fund may be prevented from holding one or more securities in the same proportion as in its target index. |
| Bar Chart and Performance Table [Heading] | rr_BarChartAndPerformanceTableHeading | <span style="color:#000000;font-family:Arial;font-size:10pt;font-weight:bold;">Annual Total Returns</span> |
| Performance Narrative [Text Block] | rr_PerformanceNarrativeTextBlock | The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund’s Institutional Plus Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Institutional Plus Shares compare with those of the Fund’s target index and comparative indexes, which have investment characteristics similar to those of the Fund. Effective July 24, 2018, the Fund began tracking the MSCI US Investable Market Real Estate 25/50 Index as its target index. The Real Estate Spliced Index reflects performance of the MSCI US REIT Index through February 1, 2018; the MSCI US Investable Market Real Estate 25/50 Transition Index through July 24, 2018; and the MSCI US Investable Market Real Estate 25/50 Index thereafter. Keep in mind that the Fund’s past performance (before and after taxes) does not indicate how the Fund will perform in the future.Updated performance information is available on our website at vanguard.com/performance or by calling Vanguard toll-free at 800-662-7447. |
| Performance Information Illustrates Variability of Returns [Text] | rr_PerformanceInformationIllustratesVariabilityOfReturns | <span style="font-family:Arial;font-size:9.5pt;">The following bar chart and table are intended to help you understand the risks of investing in the Fund. The bar chart shows how the performance of the Fund’s Institutional Plus Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Institutional Plus Shares compare with those of the Fund’s target index and comparative indexes, which have investment characteristics similar to those of the Fund.</span> |
| Performance Availability Phone [Text] | rr_PerformanceAvailabilityPhone | <span style="font-family:Arial;font-size:9.5pt;">800-662-7447</span> |
| Performance Availability Website Address [Text] | rr_PerformanceAvailabilityWebSiteAddress | <span style="font-family:Arial;font-size:9.5pt;font-style:italic;">vanguard.com/performance</span> |
| Performance Past Does Not Indicate Future [Text] | rr_PerformancePastDoesNotIndicateFuture | <span style="font-family:Arial;font-size:9.5pt;">Keep in mind that the Fund’s past performance (before </span><span style="font-family:Arial;font-size:9.5pt;">and after taxes) does not indicate how the Fund will perform in the future.</span> |
| Bar Chart [Heading] | rr_BarChartHeading | <span style="font-family:Arial;font-size:8pt;font-weight:bold;">Annual Total Returns — Vanguard Real Estate II Index Fund Institutional Plus Shares</span><span style="font-family:Arial;font-size:5pt;font-weight:bold;position:relative;top:-3.25pt;">1</span> |
| Bar Chart Closing [Text Block] | rr_BarChartClosingTextBlock | During the periods shown in the bar chart, the highest and lowest returns for a calendar quarter were: |
| Performance Table Heading | rr_PerformanceTableHeading | <span style="font-family:Arial;font-size:8.5pt;font-weight:bold;">Average Annual Total Returns for Periods Ended December 31, 2023</span> |
| Performance Table Uses Highest Federal Rate | rr_PerformanceTableUsesHighestFederalRate | <span style="font-family:Arial;font-size:9.5pt;">When after-tax returns are calculated, it is </span><span style="font-family:Arial;font-size:9.5pt;">assumed that the shareholder was in the highest individual federal marginal income tax bracket at the time of each distribution of income or capital gains or upon redemption. State and local income taxes are not reflected in the calculations. </span> |
| Performance Table Not Relevant to Tax Deferred | rr_PerformanceTableNotRelevantToTaxDeferred | <span style="font-family:Arial;font-size:9.5pt;">Please note that after-tax returns are not relevant for a shareholder </span><span style="font-family:Arial;font-size:9.5pt;">who holds fund shares in a tax-deferred account, such as an individual retirement account or a 401(k) plan. </span> |
| Performance Table One Class of after Tax Shown [Text] | rr_PerformanceTableOneClassOfAfterTaxShown | |
| Performance Table Explanation after Tax Higher | rr_PerformanceTableExplanationAfterTaxHigher | <span style="font-family:Arial;font-size:9.5pt;">Also, figures captioned </span><span style="font-family:Arial;font-size:9.5pt;font-style:italic;">Return After Taxes on Distributions and Sale of Fund Shares</span><span style="font-family:Arial;font-size:9.5pt;"> may be higher than other figures for the same period if a capital loss occurs upon redemption and results in an assumed tax deduction for the shareholder.</span> |
| Performance Table Closing [Text Block] | rr_PerformanceTableClosingTextBlock | Actual after-tax returns depend on your tax situation and may differ from those shown in the preceding table. When after-tax returns are calculated, it is assumed that the shareholder was in the highest individual federal marginal income tax bracket at the time of each distribution of income or capital gains or upon redemption. State and local income taxes are not reflected in the calculations. Please note that after-tax returns are not relevant for a shareholder who holds fund shares in a tax-deferred account, such as an individual retirement account or a 401(k) plan. Also, figures captioned Return After Taxes on Distributions and Sale of Fund Shares may be higher than other figures for the same period if a capital loss occurs upon redemption and results in an assumed tax deduction for the shareholder. |
| Institutional Plus | Vanguard Real Estate II Index Fund | Risk Lose Money [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | An investment in the Fund could lose money over short or long periods of time. |
| Institutional Plus | Vanguard Real Estate II Index Fund | Risk Not Insured Depository Institution [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | 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. |
| Institutional Plus | Vanguard Real Estate II Index Fund | Nondiversification Risk [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | • Nondiversification risk, which is the chance that the Fund may invest a greater percentage of its assets in a particular issuer or group of issuers or may own larger positions of an issuer’s voting stock than a diversified fund. |
| Institutional Plus | Vanguard Real Estate II Index Fund | Institutional Plus Shares | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Purchase Fee | rr_MaximumCumulativeSalesChargeOverOther | none |
| Sales Charge (Load) Imposed on Purchases | rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice | none |
| Sales Charge (Load) Imposed on Reinvested Dividends | rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther | none |
| Redemption Fee | rr_RedemptionFeeOverRedemption | none |
| Management Fees | rr_ManagementFeesOverAssets | 0.07% |
| 12b-1 Distribution Fee | rr_DistributionAndService12b1FeesOverAssets | none |
| Other Expenses | rr_OtherExpensesOverAssets | 0.01% |
| Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 0.08% |
| 1 Year | rr_ExpenseExampleYear01 | $ 8 |
| 3 Years | rr_ExpenseExampleYear03 | 26 |
| 5 Years | rr_ExpenseExampleYear05 | 45 |
| 10 Years | rr_ExpenseExampleYear10 | $ 103 |
| 2018 | rr_AnnualReturn2018 | (5.95%) |
| 2019 | rr_AnnualReturn2019 | 28.97% |
| 2020 | rr_AnnualReturn2020 | (4.60%) |
| 2021 | rr_AnnualReturn2021 | 40.41% |
| Annual Return 2022 | rr_AnnualReturn2022 | (26.15%) |
| 2023 | rr_AnnualReturn2023 | 11.83% |
| Year to Date Return, Label | rr_YearToDateReturnLabel | year-to-date return |
| Bar Chart, Year to Date Return, Date | rr_BarChartYearToDateReturnDate | Mar. 31, 2024 |
| Bar Chart, Year to Date Return | rr_BarChartYearToDateReturn | (1.17%) |
| Highest Quarterly Return, Label | rr_HighestQuarterlyReturnLabel | <span style="font-family:Arial;font-size:8pt;margin-left:0.0pt;">Highest</span> |
| Highest Quarterly Return, Date | rr_BarChartHighestQuarterlyReturnDate | Dec. 31, 2023 |
| Highest Quarterly Return | rr_BarChartHighestQuarterlyReturn | 18.11% |
| Lowest Quarterly Return, Label | rr_LowestQuarterlyReturnLabel | <span style="font-family:Arial;font-size:8pt;margin-left:0.0pt;">Lowest</span> |
| Lowest Quarterly Return, Date | rr_BarChartLowestQuarterlyReturnDate | Mar. 31, 2020 |
| Lowest Quarterly Return | rr_BarChartLowestQuarterlyReturn | (24.09%) |
| 1 Year | rr_AverageAnnualReturnYear01 | 11.83% |
| 5 Years | rr_AverageAnnualReturnYear05 | 7.37% |
| Since Inception | rr_AverageAnnualReturnSinceInception | 5.03% |
| Fund Inception Date | rr_AverageAnnualReturnInceptionDate | Sep. 26, 2017 |
| Institutional Plus | Vanguard Real Estate II Index Fund | Return After Taxes on Distributions | Institutional Plus Shares | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 10.64% |
| 5 Years | rr_AverageAnnualReturnYear05 | 6.29% |
| Since Inception | rr_AverageAnnualReturnSinceInception | 3.87% |
| Institutional Plus | Vanguard Real Estate II Index Fund | Return After Taxes on Distributions and Sale of Fund Shares | Institutional Plus Shares | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 7.17% |
| 5 Years | rr_AverageAnnualReturnYear05 | 5.35% |
| Since Inception | rr_AverageAnnualReturnSinceInception | 3.43% |
| Institutional Plus | Vanguard Real Estate II Index Fund | Real Estate Spliced Index (reflects no deduction for fees, expenses, or taxes) | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 11.96% |
| 5 Years | rr_AverageAnnualReturnYear05 | 7.44% |
| Since Inception | rr_AverageAnnualReturnSinceInception | 5.11% |
| Institutional Plus | Vanguard Real Estate II Index Fund | MSCI US Investable Market Real Estate 25/50 Index (reflects no deduction for fees, expenses, or taxes) | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 11.96% |
| 5 Years | rr_AverageAnnualReturnYear05 | 7.44% |
| Since Inception | rr_AverageAnnualReturnSinceInception | |
| Institutional Plus | Vanguard Real Estate II Index Fund | Dow Jones U.S. Total Stock Market Float Adjusted Index (reflects no deduction for fees, expenses, or taxes) | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 26.06% |
| 5 Years | rr_AverageAnnualReturnYear05 | 15.05% |
| Since Inception | rr_AverageAnnualReturnSinceInception | 12.15% |