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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
DFA Short-Term Extended Quality Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading DFA Short-Term Extended Quality Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the DFA Short-Term Extended Quality Portfolio (the “Short-Term Extended Quality Portfolio”) is to maximize total returns from the universe of debt securities in which the Portfolio invests. Total return is comprised of income and capital appreciation.
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 Extended Quality 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 Short-Term Extended Quality 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 25% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 25.00%
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 Short-Term Extended Quality 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 Extended Quality Portfolio seeks to maximize total returns from a universe of U.S. and foreign corporate debt securities with an investment grade credit rating. In addition, the Portfolio may invest in obligations issued or guaranteed by the U.S. and foreign governments, their agencies and instrumentalities, bank obligations, commercial paper, repurchase agreements, obligations of other domestic and foreign issuers having investment grade ratings, securities of domestic or foreign issuers denominated in U.S. dollars but not trading in the United States, and obligations of supranational organizations. The Portfolio invests with an emphasis on debt securities Dimensional Fund Advisors LP (the “Advisor”) considers to be of extended quality as they are rated in the lower half of the investment grade spectrum (e.g., rated BBB- to A+ by Standard & Poor’s Rating Group (“S&P”) or Fitch Ratings Ltd. (“Fitch”) or Baa3 to A1 by Moody’s Investor’s Service, Inc. (“Moody’s”)). The Portfolio will not emphasize investments in the lower half of the investment grade spectrum, however, when the Advisor believes the credit risk premium does not warrant the investment. The Portfolio will also invest in higher-rated debt securities. At the present time, the Advisor expects that most investments will be made in the obligations of issuers that are located in developed countries. However, in the future, the Advisor anticipates investing in issuers located in other countries as well.

The Short-Term Extended Quality Portfolio primarily invests in securities that mature within five years from the date of settlement and maintains an average portfolio maturity and an average portfolio duration of three years or less. In making these purchase decisions, if the anticipated maturity risk premium is greater for longer-term securities in the eligible maturity range, the Advisor will focus on investment in the longer-term area, otherwise, the Portfolio will focus its investment in the short-term range of the eligible maturity range. Duration is a measure of the expected life of a fixed income security that is used to determine the sensitivity of a security’s price to changes in interest rates. The longer a security’s duration, the more sensitive it will be to changes in interest rates. As a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in fixed income securities considered to be investment grade quality. In addition, 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 Short-Term Extended Quality Portfolio’s investments may include foreign securities denominated in foreign currencies. The Portfolio intends to hedge foreign currency exposure to attempt to protect against uncertainty in the level of future foreign currency rates. The Portfolio may enter into foreign forward currency contracts to hedge against fluctuations in currency exchange rates or to transfer balances from one currency to another. In regard to currency hedging, it is generally not possible to precisely match the foreign currency exposure of such forward foreign currency contracts to the value of the securities involved due to fluctuations in the market values of such securities and cash flows into and out of the Portfolio between the date a forward foreign currency contract is entered into and the date it expires. The Portfolio also may enter into credit default swaps on issuers or indices to buy or sell credit protection to hedge its credit exposure; gain market or issuer exposure without owning the underlying securities; or increase the Portfolio’s total return. The Portfolio also may purchase or sell futures contracts and options on futures contracts, to hedge its interest rate or currency exposure or for non-hedging purposes, such as a substitute for direct investment or 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.

The Short-Term Extended Quality 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. 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 Extended Quality Portfolio that owns them, to rise or fall.

Foreign Securities and Currencies Risk: Foreign securities prices may decline or fluctuate because of: (a) economic or political actions of foreign governments, and/or (b) less regulated or liquid securities markets. Investors holding these securities may also be exposed to foreign currency risk (the possibility that foreign currency will fluctuate in value against the U.S. dollar or that a foreign government will convert, or be forced to convert, its currency to another currency, changing its value against the U.S. dollar). The Short-Term Extended Quality Portfolio hedges foreign currency risk.

Foreign Government Debt Risk: The risk that: (a) the governmental entity that controls the repayment of government debt may not be willing or able to repay the principal and/or to pay the interest when it becomes due, due to factors such as political considerations, the relative size of the governmental entity’s debt position in relation to the economy, cash flow problems, insufficient foreign currency reserves, the failure to put in place economic reforms required by the International Monetary Fund or other multilateral agencies, and/or other national economic factors; (b) governments may default on their debt securities, which may require holders of such securities to participate in debt rescheduling; and (c) there is no legal or bankruptcy process by which defaulted government debt may be collected in whole or in part.

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 Short-Term Extended Quality 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. Credit risk is greater for fixed income securities with ratings below investment grade (e.g., BB+ or below by S&P or Ba1 or below by Moody’s). Fixed income securities that are below investment grade involve high credit risk and are considered speculative. Below investment grade fixed income securities may also fluctuate in value more than higher quality fixed income securities and, during periods of market volatility, may be more difficult to sell at the time and price the Portfolio desires.

Income Risk: Income risk is the risk that falling interest rates will cause the Short-Term Extended Quality 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 and foreign exchange forward contracts, whose value is derived from that of other assets, rates or indices. Derivatives can be used for hedging (attempting to reduce risk by offsetting one investment position with another) or non-hedging purposes. 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 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 Extended Quality 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. Additional risks are associated with the use of credit default swaps including counterparty and credit risk (the risk that the other party to a swap agreement will not fulfill its contractual obligations, whether because of bankruptcy or other default) and liquidity risk (the possible lack of a secondary market for the swap agreement).

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Short-Term Extended Quality 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 Extended Quality Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Short-Term Extended Quality 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 Short-Term Extended Quality 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 Short-Term Extended Quality 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.
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 Extended Quality 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 since inception 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 Extended Quality 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 Extended Quality 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 Short-Term Extended Quality Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2010-December 2016

Highest Quarter    Lowest Quarter
2.42% (7/10–9/10)
    -1.08% (10/16-12/16)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2016
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Short-Term Extended Quality 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 Extended Quality Portfolio | Institutional Class Shares  
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
2010 rr_AnnualReturn2010 5.15%
2011 rr_AnnualReturn2011 2.93%
2012 rr_AnnualReturn2012 3.63%
2013 rr_AnnualReturn2013 0.41%
2014 rr_AnnualReturn2014 1.68%
2015 rr_AnnualReturn2015 1.15%
2016 rr_AnnualReturn2016 2.06%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2010
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 2.42%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (1.08%)
1 Year rr_AverageAnnualReturnYear01 2.06%
5 Years rr_AverageAnnualReturnYear05 1.78%
Since Inception rr_AverageAnnualReturnSinceInception 3.14%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 04, 2009
DFA Short-Term Extended Quality Portfolio | Return After Taxes on Distributions | Institutional Class Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.28%
5 Years rr_AverageAnnualReturnYear05 1.07%
Since Inception rr_AverageAnnualReturnSinceInception 2.31%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 04, 2009
DFA Short-Term Extended Quality Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.17%
5 Years rr_AverageAnnualReturnYear05 1.07%
Since Inception rr_AverageAnnualReturnSinceInception 2.11%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 04, 2009
DFA Short-Term Extended Quality Portfolio | The BofA Merrill Lynch 1-5 Year US Corporate & Government Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.62% [1]
5 Years rr_AverageAnnualReturnYear05 1.39% [1]
Since Inception rr_AverageAnnualReturnSinceInception 2.47% [1]
[1] Source Merrill Lynch, used with permission. MERRILL LYNCH IS LICENSING THE MERRILL LYNCH INDICES “AS IS,” MAKES NO WARRANTIES REGARDING SAME, DOES NOT GUARANTEE THE ACCURACY AND OR COMPLETENESS OF THE MERRILL LYNCH INDICES OR ANY DATA INCLUDED THEREIN OR DERIVED THEREFROM, AND ASSUMES NO LIABILITY IN CONNECTION WITH THEIR USE.