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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2016
DFA Short-Duration Real Return Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading DFA Short-Duration Real Return Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the DFA Short-Duration Real Return Portfolio (the “Short-Duration Real Return Portfolio”) is to seek inflation protection and maximize total returns.
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-Duration Real Return 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)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination February 28, 2017
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The 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 Portfolio’s performance. During the most recent fiscal year, the Short-Duration Real Return Portfolio’s portfolio turnover rate was 30% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 30.00%
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-Duration Real Return 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 and then redeem all of your shares at the end of those periods. 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-Duration Real Return Portfolio pursues its investment objective by investing in a combination of debt securities, including inflation-protected securities, and derivative instruments. The Short-Duration Real Return Portfolio will maintain an average portfolio duration of three years or less. In making purchase decisions, if the anticipated maturity risk 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. The fixed income securities in which the Short-Duration Real Return Portfolio directly or indirectly invests are considered investment grade at the time of purchase.

The Short-Duration Real Return Portfolio may invest in inflation protected securities, obligations issued or guaranteed by the U.S. and foreign governments, their agencies and instrumentalities, corporate debt obligations, bank obligations, commercial paper, repurchase agreements, obligations of other domestic and foreign issuers, securities of domestic or foreign issuers denominated in U.S. dollars but not trading in the United States, and obligations of supranational organizations. 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-Duration Real Return Portfolio may enter into swaps, such as inflation swaps, to seek inflation protection. 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 use derivatives, such as 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 may use forward foreign currency contracts to attempt to protect against uncertainty in the level of future foreign currency rates, 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 Short-Duration Real Return Portfolio may lend its portfolio securities to generate additional income.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock 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-Duration Real Return Portfolio that owns them, to rise or fall. Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.

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).

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.

Inflation-Protected Securities Interest Rate Risk: Inflation-protected securities may react differently from other fixed income securities to changes in interest rates. Because interest rates on inflation-protected securities are adjusted for inflation, the values of these securities are not materially affected by inflation expectations. Therefore, the value of inflation-protected securities are anticipated to change in response to changes in “real” interest rates, which represent nominal (stated) interest rates reduced by the expected impact of inflation. Generally, the value of an inflation-protected security will fall when real interest rates rise and will rise when real interest rates fall.

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 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.

Risks of Investing for Inflation Protection: Because the interest and/or principal payments on an inflation-protected security are adjusted periodically for changes in inflation, the income distributed by the Portfolio may be irregular. Although the U.S. Treasury guarantees to pay at least the original face value of any inflation-protected securities the Treasury issues, other issuers may not offer the same guarantee. Also, inflation-protected securities, including those issued by the U.S. Treasury, are not protected against deflation. As a result, in a period of deflation, the principal and income of inflation-protected securities held by the Portfolio will decline and the Portfolio may suffer a loss during such periods. While inflation-protected securities are expected to be protected from long-term inflationary trends, short-term increases in inflation may lead to a decline in the Portfolio’s value. For example, if interest rates rise due to reasons other than inflation, the Portfolio’s investment in these securities may not be protected to the extent that the increase is not reflected in the securities’ inflation measures. In addition, positive adjustments to principal generally will result in taxable income to the Portfolio at the time of such adjustments (which generally would be distributed by the Portfolio as part of its taxable dividends), even though the principal amount is not paid until maturity. The current market value of inflation-protected securities is not guaranteed and will fluctuate.

Income Risk: Income risk is the risk that falling interest rates will cause the Short-Duration Real Return 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 swaps, 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 more speculative than other types of investments. When the Short-Duration Real Return Portfolio uses derivatives, it will be 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). Credit risk increases when the Portfolio is the seller of credit default swaps and counterparty risk increases when the Portfolio is a buyer of credit default swaps. In addition, where the Portfolio is the seller credit default swaps, it may be required to liquidate portfolio securities at inopportune times in order to meet payment obligations or segregation requirements. Credit default swaps may be illiquid or difficult to value.

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Short-Duration Real Return Portfolio holds illiquid investments, the Short-Duration Real Return 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-Duration Real Return Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Short-Duration Real Return 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-Duration Real Return 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-Duration Real Return 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-Duration Real Return 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 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-Duration Real Return 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-deferred 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-Duration Real Return 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-Duration Real Return Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2014-December 2015

Highest Quarter    Lowest Quarter
2.00% (4/14–6/14)    -1.94% (10/14–12/14)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2015
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Short-Duration Real Return 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-deferred arrangements, such as 401(k) plans or individual retirement accounts.
DFA Short-Duration Real Return Portfolio | Institutional Class  
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 0.03%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.23%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.01%) [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.24%
1 YEAR rr_ExpenseExampleYear01 $ 25
3 YEARS rr_ExpenseExampleYear03 75
5 YEARS rr_ExpenseExampleYear05 131
10 YEARS rr_ExpenseExampleYear10 $ 294
2014 rr_AnnualReturn2014 (0.95%)
2015 rr_AnnualReturn2015 0.46%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2014
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 2.00%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2014
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (1.94%)
1 YEAR rr_AverageAnnualReturnYear01 0.46%
SINCE INCEPTION rr_AverageAnnualReturnSinceInception (0.56%)
INCEPTION DATE rr_AverageAnnualReturnInceptionDate Nov. 03, 2013
DFA Short-Duration Real Return Portfolio | Return After Taxes on Distributions | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 YEAR rr_AverageAnnualReturnYear01 (0.10%)
SINCE INCEPTION rr_AverageAnnualReturnSinceInception (1.03%)
INCEPTION DATE rr_AverageAnnualReturnInceptionDate Nov. 03, 2013
DFA Short-Duration Real Return Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 YEAR rr_AverageAnnualReturnYear01 0.26%
SINCE INCEPTION rr_AverageAnnualReturnSinceInception (0.64%)
INCEPTION DATE rr_AverageAnnualReturnInceptionDate Nov. 03, 2013
DFA Short-Duration Real Return Portfolio | Barclays U.S. Treasury TIPS 1-5 Years (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 YEAR rr_AverageAnnualReturnYear01 (0.15%)
SINCE INCEPTION rr_AverageAnnualReturnSinceInception (0.74%)
[1] The Advisor has agreed to waive certain fees and in certain instances, assume certain expenses of the Short-Duration Real Return Portfolio. The Fee Waiver and Expense Assumption Agreement for the Short-Duration Real Return Portfolio will remain in effect through February 28, 2017, and may only be terminated by the Fund's Board of Directors prior to that date. Under certain circumstances, the Advisor retains the right to seek reimbursement for any fees previously waived and/or expenses previously assumed up to thirty-six months after such fee waiver and/or expense assumption.