v3.24.4
Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName VANGUARD CMT FUNDS
Prospectus Date rr_ProspectusDate Dec. 20, 2024
Investor | Vanguard Municipal Low Duration 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 current income that is exempt from federal personal income taxes, while maintaining liquidity.
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;">(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;">(Expenses that you pay each year as a percentage of the value of your investment)</span>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <span style="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 year ended August 31, the Fund’s portfolio turnover rate was 0% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate none
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 invests in a variety of high-quality, short-term municipal securities. To be considered high quality, a security must be determined by Vanguard to present minimal credit risk based in part on a consideration of maturity, portfolio diversification, portfolio liquidity, and credit quality. As a matter of fundamental policy, the Fund will invest at least 80% of its assets in tax-exempt municipal bonds (including securities that may be subject to alternative minimum tax) under normal market conditions. Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to interest rate movements. For example, if a bond has a duration of two years, its price would fall by approximately 2% when interest rates rise by 1%. On the other hand, the bond's price would rise by approximately 2% when interest rates fall by 1%.

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 An investment in the Fund could lose money over short or long periods of time. The Fund is designed for investors with a low tolerance for risk; however, the Fund is subject to the following risks, which could affect the Fund’s performance, and the level of risk may vary based on market conditions:• Income risk, which is the chance that the Fund’s income will decline because of falling interest rates. Because the Fund’s income is based on short-term interest rates—which can fluctuate significantly over short periods—income risk is expected to be high. A low or negative interest rate environment will adversely affect the Fund’s return. Low or negative interest rates, depending on their duration and severity, could prevent the Fund from, among other things, providing a positive yield.• Interest rate risk, which is the chance that bond prices overall will decline because of rising interest rates. 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.• Call risk, which is the chance that during periods of falling interest rates, issuers of callable bonds may call (redeem) securities with higher coupon rates or interest rates before their maturity dates. The Fund would then lose any price appreciation above the bond’s call price and would be forced to reinvest the unanticipated proceeds at lower interest rates, resulting in a decline in the Fund’s income. Such redemptions and subsequent reinvestments would also increase the Fund’s portfolio turnover rate. Call risk is generally low for short-term bond funds.• Extension risk, which is the chance that during periods of rising interest rates, certain debt securities will be paid off substantially more slowly than originally anticipated, and the value of those securities may fall. Extension risk is generally low for short-term bond funds.• Credit risk, which is the chance that the issuer of a security will fail to pay interest or principal in a timely manner or that negative perceptions of the issuer’s ability to make such payments will cause the price of that security to decline. Credit risk should be very low for the Fund because it invests primarily in securities that are considered to be high quality.• Liquidity risk, which is the chance that the Fund may not be able to sell a security, including restricted securities, 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.• Tax risk, which is the chance that all or a portion of the tax-exempt income from municipal bonds held by the Fund will be declared taxable, possibly with retroactive effect, because of unfavorable changes in tax laws, adverse interpretations by the Internal Revenue Service or state or local tax authorities, or noncompliant conduct of a bond issuer.• Structured products risk. The Fund may invest in structured products such as tender option bonds and long-term municipal bonds combined with a demand feature (i.e., variable rate demand notes or VRDNs), which may involve risks different from, and possibly greater than, those of investments directly in the underlying securities or assets.• Regional risk, which is the chance that economic, political, or regulatory occurrences within a certain state may adversely affect the value of securities offered by issuers located within that state. Because the Fund may invest a large portion of its assets in securities located in any one state, the Fund’s performance may be hurt disproportionately by the poor performance of its investments in that area.
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 has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Fund compare with those of a comparative benchmark, which has investment characteristics similar to those of the Fund. Returns for the Short Municipal Debt Funds Average are derived from data provided by Lipper, a Thomson Reuters Company. The Fund is using a benchmark that is different from the one used for the immediately preceding fiscal year to reflect that the Fund is currently a municipal short-term bond fund, whereas in the preceding fiscal year, the Fund had been a municipal money market fund. Information regarding the former benchmark, the Tax-Exempt Money Market Funds Average, derived from data provided by Lipper, a Thomson Reuters Company, is also provided. 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.
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 has varied from one calendar year to another over the periods shown. The table shows how the average annual total returns of the Fund compare with those of a comparative benchmark, which has investment characteristics similar to those of the Fund.</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 </span><span style="font-family:Arial;font-size:9.5pt;">performance (before 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 Municipal Low Duration 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="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;">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. </span>
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown <span style="font-family:Arial;font-size:9.5pt;">After-tax returns are </span>
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. 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.
Investor | Vanguard Municipal Low Duration 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.
Investor | Vanguard Municipal Low Duration 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
Management Fees rr_ManagementFeesOverAssets 0.01%
12b-1 Distribution Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets none
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.01%
1 Year rr_ExpenseExampleYear01 $ 1
3 Years rr_ExpenseExampleYear03 3
5 Years rr_ExpenseExampleYear05 6
10 Years rr_ExpenseExampleYear10 $ 13
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2024
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 2.49%
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 0.89%
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, 2015
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn 0.01%
1 Year rr_AverageAnnualReturnYear01 3.30%
5 Years rr_AverageAnnualReturnYear05 1.28%
10 Years rr_AverageAnnualReturnYear10 0.91%
Investor | Vanguard Municipal Low Duration Fund | Return After Taxes on Distributions | Investor Shares  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.94%
5 Years rr_AverageAnnualReturnYear05 0.76%
10 Years rr_AverageAnnualReturnYear10 0.54%
Investor | Vanguard Municipal Low Duration Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor Shares  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.94%
5 Years rr_AverageAnnualReturnYear05 0.76%
10 Years rr_AverageAnnualReturnYear10 0.54%
Investor | Vanguard Municipal Low Duration Fund | Tax-Exempt Money Market Funds Average  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.96%
5 Years rr_AverageAnnualReturnYear05 1.04%
10 Years rr_AverageAnnualReturnYear10 0.67%
Investor | Vanguard Municipal Low Duration Fund | Short Municipal Debt Funds Average  
Risk Return Abstract rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 3.62%
5 Years rr_AverageAnnualReturnYear05 1.22%
10 Years rr_AverageAnnualReturnYear10 0.99%