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Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2013
VA SHORT-TERM FIXED PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading VA Short-Term Fixed Portfolio
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the VA Short-Term Fixed Portfolio is to achieve a stable real return in excess of the rate of inflation with a minimum of risk.
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 VA Short-Term Fixed Portfolio. The expenses in the table do not include any fees or charges imposed by the variable insurance contract. If such fees and charges were included, the expenses would be higher.
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 VA Short-Term Fixed 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 VA Short-Term Fixed Portfolio’s performance. During the most recent fiscal year, the VA Short-Term Fixed Portfolio’s portfolio turnover rate was 85% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 85.00%
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 VA Short-Term Fixed Portfolio with the cost of investing in other mutual funds. The Example does not include any fees or charges imposed by the variable insurance contract. 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 Dimensional Fund Advisors LP (the "Advisor") believes that fixed income investing should involve a long-term view and a systematic focus on bond market risk and return, not on interest rate forecasting or market timing.

In constructing an investment portfolio, the Advisor identifies a broadly diversified universe of eligible securities with precisely defined maturity ranges and credit quality characteristics. The Advisor will then seek to purchase a broad and diverse portfolio of securities meeting these credit quality standards. 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 investment in that longer-term area, otherwise, the portfolio will focus investment in the short-term range of the eligible maturity range. The Advisor also places priority on efficiently managing portfolio turnover and keeping trading costs low.

The VA Short-Term Fixed Portfolio seeks to achieve its investment objective by generally investing in a universe of high quality fixed income securities that typically mature in one year or less. The Portfolio may, however, take a large position in securities maturing within two years from the date of settlement when higher yields are available. The Portfolio purchases U.S. government obligations, U.S. government agency obligations, dollar-denominated obligations of foreign issuers issued in the U.S., foreign government and agency obligations, bank obligations, including U.S. subsidiaries and branches of foreign banks, corporate obligations, commercial paper, repurchase agreements, obligations of supranational organizations and affiliated and unaffiliated unregistered money market funds. As a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in fixed income securities and maintain a dollar-weighted average portfolio maturity that will not exceed one year. The Portfolio principally invests in certificates of deposit, commercial paper, bankers' acceptances, notes and bonds.

The VA Short-Term Fixed Portfolio may concentrate its investments in obligations of U.S. and foreign banks and bank holding companies. The Portfolio will invest more than 25% of its total assets in obligations of U.S. and/or foreign banks and bank holding companies ("banking industry securities") when the yield to maturity on eligible portfolio investments in banking industry securities as a group generally exceeds the yield to maturity on all other eligible portfolio investments as a group for a period of five consecutive days when the New York Stock Exchange is open for trading. See the section entitled "Investments in the Banking Industry—VA Short-Term Fixed Portfolio" in the Portfolio's prospectus for additional information.

The VA Short-Term Fixed 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 VA Short-Term Fixed 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 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.

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 VA International Value Portfolio does not hedge foreign currency risk.

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 these price changes.

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 VA Short-Term Fixed 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 VA Short-Term Fixed Portfolio's income to decline.

Risks of Banking Concentration: Focus on the banking industry would link the performance of the VA Short-Term Fixed Portfolio to changes in performance of the banking industry generally. For example, a change in the market's perception of the riskiness of banks compared to non-banks would cause the value of the Portfolio's securities to fluctuate. Banks are very sensitive to changes in money market and general economic conditions. The profitability of the banking industry is dependent upon banks being able to obtain funds at reasonable costs and upon liquidity in the capital and credit markets to finance their lending operations. Adverse general economic conditions can cause financial difficulties for a bank's borrowers and the borrowers' failure to repay their loans can adversely affect the bank's financial situation. Banks are subject to extensive regulation and decisions by regulators may limit the loans banks make and the interest rates and fees they charge, which could reduce bank profitability.

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 VA Short-Term Fixed 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.
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 VA Short-Term Fixed 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 performance reflected in the bar chart for the Portfolio does not reflect any insurance company separate account charges, which if reflected would lower returns. 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 www.dimensional.com.

The after-tax returns presented in the table for the VA Short-Term Fixed 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 VA Short-Term Fixed 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 www.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 VA Short-Term Fixed Portfolio
Total Returns (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2003-December 2012

Highest Quarter    Lowest Quarter
2.34 (10/08-12/08)    -0.59 (4/04-6/04)
Performance Table Heading rr_PerformanceTableHeading Periods ending December 31, 2012
Annualized Returns (%)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the VA Short-Term Fixed 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
VA SHORT-TERM FIXED PORTFOLIO | VA SHORT-TERM FIXED PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.05%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.30%
1 Year rr_ExpenseExampleYear01 31
3 Years rr_ExpenseExampleYear03 97
5 Years rr_ExpenseExampleYear05 169
10 Years rr_ExpenseExampleYear10 381
2003 rr_AnnualReturn2003 1.40%
2004 rr_AnnualReturn2004 0.78%
2005 rr_AnnualReturn2005 2.18%
2006 rr_AnnualReturn2006 4.63%
2007 rr_AnnualReturn2007 4.98%
2008 rr_AnnualReturn2008 3.96%
2009 rr_AnnualReturn2009 1.86%
2010 rr_AnnualReturn2010 1.11%
2011 rr_AnnualReturn2011 0.43%
2012 rr_AnnualReturn2012 0.84%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 2.34%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Jun. 30, 2004
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.59%)
One Year rr_AverageAnnualReturnYear01 0.84%
Five Years rr_AverageAnnualReturnYear05 1.63%
Ten Years rr_AverageAnnualReturnYear10 2.20%
VA SHORT-TERM FIXED PORTFOLIO | Return After Taxes on Distributions | VA SHORT-TERM FIXED PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.64%
Five Years rr_AverageAnnualReturnYear05 1.16%
Ten Years rr_AverageAnnualReturnYear10 1.46%
VA SHORT-TERM FIXED PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | VA SHORT-TERM FIXED PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.57%
Five Years rr_AverageAnnualReturnYear05 1.14%
Ten Years rr_AverageAnnualReturnYear10 1.46%
VA SHORT-TERM FIXED PORTFOLIO | BofA Merrill Lynch US 6-Month Treasury Bill Index (reflects no deduction for fees, expenses, or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.17% [1]
Five Years rr_AverageAnnualReturnYear05 0.98% [1]
Ten Years rr_AverageAnnualReturnYear10 2.08% [1]
VA SHORT-TERM FIXED PORTFOLIO | BofA Merrill Lynch 1-Year US Treasury Note Index (reflects no deduction for fees, expenses, or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.24% [1]
Five Years rr_AverageAnnualReturnYear05 1.42% [1]
Ten Years rr_AverageAnnualReturnYear10 2.19% [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 QUALITY, 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.