v3.24.1.1.u2
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 Long-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 Long-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 a high and sustainable level of current income.
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 136% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 136.00%
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 15 to 30 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 • Interest rate risk, which is the chance that bond prices overall will decline because of rising interest rates. Interest rate risk should be high for the Fund because it invests primarily in long-term bonds, whose prices are more sensitive to interest rate changes than are the prices of shorter-term bonds.• Income risk, which is the chance that the Fund’s income will decline because of falling interest rates. Income risk should be low for the Fund because it invests primarily in long-term bonds.• Liquidity risk, which is the chance that the Fund may not be able to sell a security in a timely manner at a desired price.• 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 Long-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 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 Long-Term Treasury 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.
Retail | Vanguard Long-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 Long-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 0.18%
12b-1 Distribution Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.02%
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 (3.25%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="color:#000000;font-family:Arial;font-size:8pt;margin-left:0.0pt;">Highest</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 20.84%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="color:#000000;font-family:Arial;font-size:8pt;margin-left:0.0pt;">Lowest</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2021
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (13.28%)
1 Year rr_AverageAnnualReturnYear01 3.31%
5 Years rr_AverageAnnualReturnYear05 (1.32%)
10 Years rr_AverageAnnualReturnYear10 2.21%
Retail | Vanguard Long-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 3.41%
5 Years rr_AverageAnnualReturnYear05 (1.22%)
10 Years rr_AverageAnnualReturnYear10 2.31%
Retail | Vanguard Long-Term Treasury Fund | Return After Taxes on Distributions | Investor Shares  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.90%
5 Years rr_AverageAnnualReturnYear05 (2.89%)
10 Years rr_AverageAnnualReturnYear10 0.63%
Retail | Vanguard Long-Term Treasury Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor Shares  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.93%
5 Years rr_AverageAnnualReturnYear05 (1.28%)
10 Years rr_AverageAnnualReturnYear10 1.29%
Retail | Vanguard Long-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 Long-Term Treasury Fund | Bloomberg U.S. Long Treasury Bond Index (reflects no deduction for fees, expenses, or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 3.06%
5 Years rr_AverageAnnualReturnYear05 (1.24%)
10 Years rr_AverageAnnualReturnYear10 2.28%