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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2019
DFA SHORT-TERM MUNICIPAL BOND PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading DFA Short-Term Municipal Bond Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the DFA Short-Term Municipal Bond Portfolio (the “Short-Term Municipal Bond Portfolio”) is to provide current income that is exempt from federal personal income taxes and to preserve investors’ principal.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Portfolio
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses you may pay if you buy and hold shares of the Short-Term Municipal Bond Portfolio.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <b>Shareholder Fees (fees paid directly from your investment):</b>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <b>Annual Fund Operating Expenses (expenses that you pay each<br/>year as a percentage of the value of your investment) </b>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <b>PORTFOLIO TURNOVER </b>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Short-Term Municipal Bond Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Portfolio shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Short-Term Municipal Bond Portfolio’s performance. During the most recent fiscal year, the Short-Term Municipal Bond Portfolio’s portfolio turnover rate was 31% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 31.00%
Expense Example [Heading] rr_ExpenseExampleHeading <b>EXAMPLE </b>
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is meant to help you compare the cost of investing in the Short-Term Municipal Bond Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Short-Term Municipal Bond Portfolio seeks its investment objective by investing primarily in a universe of investment grade municipal securities, the interest on which is exempt from regular federal income tax. Municipal securities include bonds, notes, commercial paper and other instruments (including participation interests in such securities) issued by or on behalf of the states, territories and possessions of the United States (including the District of Columbia) and their political subdivisions, agencies and instrumentalities. The interest on the municipal securities purchased by the Portfolio, in the opinion of bond counsel for the issuers and under current tax law, is exempt from federal income tax (i.e., excludable from gross income for individuals for federal income tax purposes but not necessarily exempt from state or local taxes). As a fundamental investment policy, under normal market conditions, the Portfolio will invest at least 80% of its net assets in municipal securities that pay interest exempt from federal income tax. The Portfolio does not currently intend to invest its assets in municipal securities whose interest is subject to the federal alternative minimum tax.

Generally, the Short-Term Municipal Bond Portfolio will acquire obligations that mature within three years from the date of settlement, but substantial investments may be made in obligations maturing up to ten years from the date of settlement when greater expected returns are available, and in variable rate demand notes with longer maturities. Under normal circumstances, the Portfolio will maintain a dollar-weighted average portfolio maturity of three years or less. In making purchase decisions, if the expected term premium is greater for longer-term securities in the eligible maturity range, Dimensional Fund Advisors LP (the “Advisor”) will focus investment in the longer-term area, otherwise, the Portfolio will focus investment in the shorter-term area of the eligible maturity range. If a security has been redeemed by the issuer at a date prior to the stated final maturity date for the purposes of the above maturity restriction, the early redemption date shall be considered the maturity date regardless of the stated final maturity.

At least 75% of the assets of the Short-Term Municipal Bond Portfolio will be invested in municipal securities that, at the time of purchase, are rated in the top three credit-rating categories (e.g., Aaa, Aa and A for Moody’s Investor’s Service, Inc. (“Moody’s”) or AAA, AA and A for S&P Global Ratings (“S&P”) or AAA, AA and A for Fitch Ratings Ltd. (“Fitch”)). No more than 25% of the Portfolio’s assets will be invested in municipal securities that, at the time of purchase, are rated in the lowest quarter of the investment grade spectrum (e.g., rated Baa3 to Baa1 (by Moody’s) or BBB- to BBB+ (by S&P or Fitch), or an equivalent rating assigned by another nationally recognized statistical rating organization, or that are unrated but have been determined by the Advisor to be of comparable quality). The fixed income securities in which the Short-Term Municipal Bond Portfolio invests are considered investment grade at the time of purchase.

The Short-Term Municipal Bond Portfolio may (1) purchase certain municipal securities that are insured, (2) invest in municipal securities secured by mortgages on single-family homes and multi-family projects, (3) invest in pre-refunded municipal securities, (4) purchase tax-exempt municipal securities on a “when-issued” basis, and (5) use fixed income related futures and options contracts, credit default swaps, interest rate swaps and other types of derivatives to hedge against changes in interest rates. The Portfolio may also purchase or sell futures contracts and options on futures contracts, to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio. The Portfolio does not intend to sell futures contracts to establish short positions in individual securities or to use derivatives for purposes of speculation or leveraging investment returns.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money. An investment in the Portfolio is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The following is a description of principal risks of investing in the Portfolio.

Market Risk: Even a long-term investment approach cannot guarantee a profit. Economic, political, and issuer-specific events will cause the value of securities, and the Short-Term Municipal Bond Portfolio that owns them, to rise or fall.

Interest Rate Risk: Fixed income securities are subject to interest rate risk because the prices of fixed income securities tend to move in the opposite direction of interest rates. When interest rates rise, fixed income security prices fall. When interest rates fall, fixed income security prices rise. In general, fixed income securities with longer maturities are more sensitive to changes in interest rates.

Credit Risk: Credit risk is the risk that the issuer of a security, including a governmental entity, may be unable to make interest payments and/or repay principal when due. A downgrade to an issuer’s credit rating or a perceived change in an issuer’s financial strength may affect a security’s value, and thus, impact the Short-Term Municipal Bond Portfolio’s performance. The ability of a municipal securities issuer to make payments could be affected by litigation, legislation or other political events or the bankruptcy of the issuer.

Income Risk: Income risk is the risk that falling interest rates will cause the Short-Term Municipal Bond Portfolio’s income to decline because, among other reasons, the proceeds from maturing short-term securities in its portfolio may be reinvested in lower-yielding securities.

Tax Liability Risk: Tax liability risk is the risk that distributions by the Short-Term Municipal Bond Portfolio become taxable to shareholders due to noncompliant conduct by a municipal bond issuer, unfavorable changes in federal or state tax laws, or adverse interpretations of tax laws by the Internal Revenue Service or state tax authorities or other factors. Such adverse interpretations or actions could cause interest from a security to become taxable, possibly retroactively, subjecting, shareholders to increased tax liability. In addition, such adverse interpretations or actions could cause the value of a security, and therefore, the value of the Portfolio’s shares, to decline.

Derivatives Risk: Derivatives are instruments, such as futures contracts, and options thereon, whose value is derived from that of other assets, rates or indices. Hedging with derivatives may increase expenses, and there is no guarantee that a hedging strategy will work. While hedging can reduce or eliminate losses, it can also reduce or eliminate gains or cause losses if the market moves in a manner different from that anticipated by the Short-Term Municipal Bond Portfolio or if the cost of the derivative outweighs the benefit of the hedge. The use of derivatives for non-hedging purposes may be considered to carry more risk than other types of investments. When the Short-Term Municipal Bond Portfolio uses derivatives, the Portfolio will be directly exposed to the risks of those derivatives. Derivative instruments are subject to a number of risks including counterparty, liquidity, interest rate, market, credit and management risks, as well as the risk of improper valuation. Changes in the value of a derivative may not correlate perfectly with the underlying asset, rate or index, and the Portfolio could lose more than the principal amount invested. Additionally, payments made or received by the Short-Term Municipal Bond Portfolio under such derivatives may increase the amount of distributions taxable to you as ordinary income, increase or decrease the amount of capital gain distributions to you and/or decrease the amount available for distribution to you as exempt-interest dividends.

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Short-Term Municipal Bond Portfolio holds illiquid investments, the Portfolio’s performance may be reduced due to an inability to sell the investments at opportune prices or times. Liquid portfolio investments may become illiquid or less liquid after purchase by the Short-Term Municipal Bond Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Short-Term Municipal Bond Portfolio will experience significant net redemptions at a time when it cannot find willing buyers for its portfolio securities or can only sell its portfolio securities at a material loss. Liquidity risk can be more pronounced in periods of market turmoil.

Cyber Security Risk: The Short-Term Municipal Bond Portfolio’s and its service providers’ use of internet, technology and information systems may expose the Portfolio to potential risks linked to cyber security breaches of those technological or information systems. Cyber security breaches, amongst other things, could allow an unauthorized party to gain access to proprietary information, customer data, or fund assets, or cause the Portfolio and/or its service providers to suffer data corruption or lose operational functionality.
Risk Lose Money [Text] rr_RiskLoseMoney Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the Portfolio 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 Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table immediately following illustrate the variability of the Short-Term Municipal Bond Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in the Portfolio’s performance from year to year. The table illustrates how annualized one year, five year and ten year returns, both before and after taxes, compare with those of a broad measure of market performance. The Portfolio’s past performance (before and after taxes) is not an indication of future results. Updated performance information for the Portfolio can be obtained by visiting http://us.dimensional.com.

The after-tax returns presented in the table for the Short-Term Municipal Bond Portfolio are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown in the table. In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table immediately following illustrate the variability of the Short-Term Municipal Bond Portfolio’s returns and are meant to provide some indication of the risks of investing in the Portfolio. The bar chart shows the changes in the Portfolio’s performance from year to year.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress http://us.dimensional.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading <b>DFA Short-Term Municipal Bond Portfolio Institutional Class Shares—Total Returns </b>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2009-December 2018

Highest Quarter    Lowest Quarter
1.40% (7/09–9/09)
    -0.94% (10/16–12/16)
Performance Table Heading rr_PerformanceTableHeading <b>Annualized Returns (%) </b><br/>Periods ending December 31, 2018
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Short-Term Municipal Bond Portfolio are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred In addition, the after-tax returns shown are not relevant to investors who hold shares of the Portfolio through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts.
DFA SHORT-TERM MUNICIPAL BOND PORTFOLIO | DFA Short-Term Municipal Bond Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.20%
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.22%
1 Year rr_ExpenseExampleYear01 $ 23
3 Years rr_ExpenseExampleYear03 71
5 Years rr_ExpenseExampleYear05 124
10 Years rr_ExpenseExampleYear10 280
1 Year rr_ExpenseExampleNoRedemptionYear01 23
3 Years rr_ExpenseExampleNoRedemptionYear03 71
5 Years rr_ExpenseExampleNoRedemptionYear05 124
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 280
2009 rr_AnnualReturn2009 3.69%
2010 rr_AnnualReturn2010 1.10%
2011 rr_AnnualReturn2011 2.38%
2012 rr_AnnualReturn2012 0.73%
2013 rr_AnnualReturn2013 0.46%
2014 rr_AnnualReturn2014 0.57%
2015 rr_AnnualReturn2015 1.15%
2016 rr_AnnualReturn2016 0.05%
2017 rr_AnnualReturn2017 1.15%
2018 rr_AnnualReturn2018 1.21%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 1.40%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.94%)
1 Year rr_AverageAnnualReturnYear01 1.21%
5 Years rr_AverageAnnualReturnYear05 0.82%
10 Years rr_AverageAnnualReturnYear10 1.24%
DFA SHORT-TERM MUNICIPAL BOND PORTFOLIO | Return After Taxes on Distributions | DFA Short-Term Municipal Bond Portfolio  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.21%
5 Years rr_AverageAnnualReturnYear05 0.82%
10 Years rr_AverageAnnualReturnYear10 1.24%
DFA SHORT-TERM MUNICIPAL BOND PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | DFA Short-Term Municipal Bond Portfolio  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.17%
5 Years rr_AverageAnnualReturnYear05 0.86%
10 Years rr_AverageAnnualReturnYear10 1.24%
DFA SHORT-TERM MUNICIPAL BOND PORTFOLIO | ICE BofAML 1-3 Year US Municipal Securities Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.76% [1]
5 Years rr_AverageAnnualReturnYear05 0.91% [1]
10 Years rr_AverageAnnualReturnYear10 1.45% [1]
[1] ICE BofAML index data copyright 2018 ICE Data Indices, LLC.