| Label | Element | Value |
|---|---|---|
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Registrant Name | dei_EntityRegistrantName | Vanguard Fixed Income Securities Funds |
| Prospectus Date | rr_ProspectusDate | May 24, 2024 |
| Retail | Vanguard Short-Term Treasury Fund | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk/Return [Heading] | rr_RiskReturnHeading | <span style="color:#000000;font-family:Arial;font-size:13pt;font-weight:bold;">Vanguard Short-Term Treasury Fund</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 current income while maintaining limited price volatility. |
| 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 Investor Shares or Admiral 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 examples 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 examples, reduce the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 314% of the average value of its portfolio. |
| Portfolio Turnover, Rate | rr_PortfolioTurnoverRate | 314.00% |
| Expense Example [Heading] | rr_ExpenseExampleHeading | <span style="color:#000000;font-family:Arial;font-size:9.5pt;">Examples</span> |
| Expense Example Narrative [Text Block] | rr_ExpenseExampleNarrativeTextBlock | The following examples are intended to help you compare the cost of investing in the Fund’s Investor Shares or Admiral Shares with the cost of investing in other mutual funds. They illustrate the hypothetical expenses that you would incur over various periods if you were to invest $10,000 in the Fund’s shares. These examples assume that the shares provide 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 invests at least 80% of its assets in U.S. Treasury securities, which include bills, bonds, and notes issued by the U.S. Treasury. The Fund is expected to maintain a dollar-weighted average maturity of 1 to 4 years. The Fund may invest in derivatives instruments, such as options, futures contracts, and other swap agreements. |
| 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 | • Income risk, which is the chance that the Fund’s income will decline because of falling interest rates. Income risk is generally high for short-term bond funds, so investors should expect the Fund’s monthly income to fluctuate accordingly.• Interest rate risk, which is the chance that bond prices overall will decline because of rising interest rates. Although inflation-indexed bonds seek to provide inflation protection, their prices may decline when interest rates rise and vice versa. Interest rate risk should be low for the Fund because it invests primarily in short-term bonds, whose prices are less sensitive to interest rate changes than are the prices of longer-term bonds.• Manager risk, which is the chance that poor security selection will cause the Fund to underperform relevant benchmarks or other funds with a similar investment objective.• Derivatives risk. The Fund may invest in derivatives, which may involve risks different from, and possibly greater than, those of investments directly in the underlying securities or assets. |
| 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 Investor Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the share classes presented compare with those of a relevant market index and another comparative index, which have investment characteristics similar to those of the Fund. 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 Investor Shares has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the share classes presented compare with those of a relevant market index and another comparative index, 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 and after taxes) </span><span style="font-family:Arial;font-size:9.5pt;">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 Short-Term Treasury Fund Investor 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="color:#000000;font-family:Arial;font-size:9.5pt;">When after-tax returns are calculated, it is </span><span style="color:#000000;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="color:#000000;font-family:Arial;font-size:9.5pt;">not relevant for </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;">a shareholder 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 | <span style="color:#000000;font-family:Arial;font-size:9.5pt;">Please note that after-tax returns are shown only for the Investor </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;">Shares and may differ for each share class. After-tax returns are </span> |
| Performance Table Explanation after Tax Higher | rr_PerformanceTableExplanationAfterTaxHigher | <span style="color:#000000;font-family:Arial;font-size:9.5pt;">Also, figures captioned </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;font-style:italic;">Return </span><span style="color:#000000;font-family:Arial;font-size:9.5pt;font-style:italic;">After Taxes on Distributions and Sale of Fund Shares</span><span style="color:#000000;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 shown only for the Investor Shares and may differ for each share class. 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. |
| Retail | Vanguard Short-Term Treasury Fund | Risk Lose Money [Member] | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| Risk [Text Block] | rr_RiskTextBlock | The Fund is designed for investors with a low tolerance for risk, but you could still lose money by investing in it. |
| Retail | Vanguard Short-Term Treasury 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. |
| Retail | Vanguard Short-Term Treasury Fund | Investor 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 |
| Account Service Fee Per Year (for certain fund account balances below $5,000,000) | rr_MaximumAccountFee | $ 25 |
| Management Fees | rr_ManagementFeesOverAssets | 0.19% |
| 12b-1 Distribution Fee | rr_DistributionAndService12b1FeesOverAssets | none |
| Other Expenses | rr_OtherExpensesOverAssets | 0.01% |
| Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 0.20% |
| 1 Year | rr_ExpenseExampleYear01 | $ 20 |
| 3 Years | rr_ExpenseExampleYear03 | 64 |
| 5 Years | rr_ExpenseExampleYear05 | 113 |
| 10 Years | rr_ExpenseExampleYear10 | $ 255 |
| 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 | 0.16% |
| 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 | 2.84% |
| 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, 2022 |
| Lowest Quarterly Return | rr_BarChartLowestQuarterlyReturn | (2.56%) |
| 1 Year | rr_AverageAnnualReturnYear01 | 3.51% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.04% |
| 10 Years | rr_AverageAnnualReturnYear10 | 0.91% |
| Retail | Vanguard Short-Term Treasury Fund | Admiral 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 |
| Account Service Fee Per Year (for certain fund account balances below $5,000,000) | rr_MaximumAccountFee | $ 25 |
| Management Fees | rr_ManagementFeesOverAssets | 0.09% |
| 12b-1 Distribution Fee | rr_DistributionAndService12b1FeesOverAssets | none |
| Other Expenses | rr_OtherExpensesOverAssets | 0.01% |
| Total Annual Fund Operating Expenses | rr_ExpensesOverAssets | 0.10% |
| 1 Year | rr_ExpenseExampleYear01 | $ 10 |
| 3 Years | rr_ExpenseExampleYear03 | 32 |
| 5 Years | rr_ExpenseExampleYear05 | 56 |
| 10 Years | rr_ExpenseExampleYear10 | $ 128 |
| 1 Year | rr_AverageAnnualReturnYear01 | 3.61% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.14% |
| 10 Years | rr_AverageAnnualReturnYear10 | 1.01% |
| Retail | Vanguard Short-Term Treasury Fund | Return After Taxes on Distributions | Investor Shares | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 1.86% |
| 5 Years | rr_AverageAnnualReturnYear05 | 0.15% |
| 10 Years | rr_AverageAnnualReturnYear10 | 0.22% |
| Retail | Vanguard Short-Term Treasury Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor Shares | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 2.06% |
| 5 Years | rr_AverageAnnualReturnYear05 | 0.43% |
| 10 Years | rr_AverageAnnualReturnYear10 | 0.40% |
| Retail | Vanguard Short-Term Treasury Fund | Bloomberg U.S. Aggregate Bond Index (reflects no deduction for fees, expenses, or taxes) | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 5.53% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.10% |
| 10 Years | rr_AverageAnnualReturnYear10 | 1.81% |
| Retail | Vanguard Short-Term Treasury Fund | Bloomberg U.S. 1-5 Year Treasury Bond Index (reflects no deduction for fees, expenses, or taxes) | ||
| Risk Return Abstract | rr_RiskReturnAbstract | |
| 1 Year | rr_AverageAnnualReturnYear01 | 4.37% |
| 5 Years | rr_AverageAnnualReturnYear05 | 1.18% |
| 10 Years | rr_AverageAnnualReturnYear10 | 1.12% |