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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2018
DFA INTERMEDIATE GOVERNMENT FIXED INCOME PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading DFA Intermediate Government Fixed Income Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the DFA Intermediate Government Fixed Income Portfolio (the “Intermediate Government Portfolio”) is to earn current income consistent with preservation of capital.
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 Intermediate Government Portfolio.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment):
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay each
year as a percentage of the value of your investment)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Intermediate Government 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 Portfolio’s performance. During the most recent fiscal year, the Portfolio’s portfolio turnover rate was 12% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 12.00%
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is meant to help you compare the cost of investing in the Intermediate Government 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 Intermediate Government Portfolio primarily invests in high quality, low-risk obligations of the U.S. Government and its agencies with maturities between five and fifteen years. The Intermediate Government Portfolio normally invests in non-callable obligations issued or guaranteed by the U.S. Government and U.S. government agencies, AAA-rated, dollar-denominated obligations of foreign governments, obligations of supranational organizations, and futures contracts on U.S. Treasury securities. Since government guaranteed mortgage-backed securities are considered callable, such securities will not be included in the Portfolio.

Generally, the Intermediate Government Portfolio will purchase securities with maturities between five and fifteen years from the date of settlement. The Portfolio will not shift the maturity of its investments in anticipation of interest rate movements and ordinarily will have an average weighted maturity, based upon market values, between three to ten years. In any event, as a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in fixed income government securities and maintain a weighted average portfolio maturity between three and ten years.

The Intermediate Government Portfolio may invest a portion of its assets in the obligations of foreign governments. Those obligations at the time of purchase must be either rated in the highest rating category of a nationally recognized statistical rating organization or, in the case of any obligation that is unrated, of comparable quality. The Portfolio is authorized to invest more than 25% of its total assets in U.S. Treasury bonds, bills and notes and obligations of federal agencies and instrumentalities. The Portfolio also may purchase or sell futures contracts on U.S. Treasury securities or options on such contracts to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio. However, the Portfolio will not purchase or sell futures contracts or options thereon if as a result more than 5% of its net assets would then consist of initial margin deposits and premiums required to establish such positions. 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.

The Intermediate Government Portfolio may lend its portfolio securities to generate additional income.
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 Intermediate Government 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 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 Intermediate Government Portfolio’s performance. Government agency obligations have different levels of credit support and, therefore, different degrees of credit risk. Securities issued by agencies and instrumentalities of the U.S. Government that are supported by the full faith and credit of the United States, such as the Federal Housing Administration and Ginnie Mae, present little credit risk. Other securities issued by agencies and instrumentalities sponsored by the U.S. Government, that are supported only by the issuer’s right to borrow from the U.S. Treasury, subject to certain limitations, and securities issued by agencies and instrumentalities sponsored by the U.S. Government that are sponsored by the credit of the issuing agencies, such as Freddie Mac and Fannie Mae, are subject to a greater degree of credit risk. U.S. government agency securities issued or guaranteed by the credit of the agency may still involve a risk of non-payment of principal and/or interest.

Income Risk: Income risk is the risk that falling interest rates will cause the Intermediate Government 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.

Derivatives Risk: Derivatives are instruments, such as futures contracts, whose value is derived from that of other assets, rates or indices. The use of derivatives for non-hedging purposes may be considered to carry more risk than other types of investments. When the Intermediate Government 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, and 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.

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Intermediate Government 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 Intermediate Government Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Intermediate Government 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.

Securities Lending Risk: Securities lending involves the risk that the borrower may fail to return the securities in a timely manner or at all. As a result, the Intermediate Government Portfolio may lose money and there may be a delay in recovering the loaned securities. The Portfolio could also lose money if it does not recover the securities and/or the value of the collateral falls, including the value of investments made with cash collateral. Securities lending also may have certain adverse tax consequences.

Cyber Security Risk: The Intermediate Government 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 Intermediate Government 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 Intermediate Government 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 Intermediate Government 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 DFA Intermediate Government Fixed Income Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2008-December 2017

Highest Quarter    Lowest Quarter
9.29% (10/08–12/08)
   -4.26% (10/16-12/16)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2017
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Intermediate Government 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 INTERMEDIATE GOVERNMENT FIXED INCOME PORTFOLIO | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.10%
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.12%
1 Year rr_ExpenseExampleYear01 $ 12
3 Years rr_ExpenseExampleYear03 39
5 Years rr_ExpenseExampleYear05 68
10 Years rr_ExpenseExampleYear10 154
1 Year rr_ExpenseExampleNoRedemptionYear01 12
3 Years rr_ExpenseExampleNoRedemptionYear03 39
5 Years rr_ExpenseExampleNoRedemptionYear05 68
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 154
2008 rr_AnnualReturn2008 12.89%
2009 rr_AnnualReturn2009 (0.72%)
2010 rr_AnnualReturn2010 6.92%
2011 rr_AnnualReturn2011 9.43%
2012 rr_AnnualReturn2012 3.71%
2013 rr_AnnualReturn2013 (3.52%)
2014 rr_AnnualReturn2014 5.18%
2015 rr_AnnualReturn2015 1.77%
2016 rr_AnnualReturn2016 1.15%
2017 rr_AnnualReturn2017 2.22%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 9.29%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (4.26%)
1 Year rr_AverageAnnualReturnYear01 2.22%
5 Years rr_AverageAnnualReturnYear05 1.32%
10 Years rr_AverageAnnualReturnYear10 3.80%
DFA INTERMEDIATE GOVERNMENT FIXED INCOME PORTFOLIO | Return After Taxes on Distributions | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.31%
5 Years rr_AverageAnnualReturnYear05 0.33%
10 Years rr_AverageAnnualReturnYear10 2.60%
DFA INTERMEDIATE GOVERNMENT FIXED INCOME PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.27%
5 Years rr_AverageAnnualReturnYear05 0.58%
10 Years rr_AverageAnnualReturnYear10 2.51%
DFA INTERMEDIATE GOVERNMENT FIXED INCOME PORTFOLIO | Bloomberg Barclays U.S. Government Bond Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 2.30%
5 Years rr_AverageAnnualReturnYear05 1.28%
10 Years rr_AverageAnnualReturnYear10 3.23%