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Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2013
JAPANESE SMALL COMPANY PORTFOLIO
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Japanese Small Company Portfolio
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the Japanese Small Company Portfolio is to achieve long-term capital appreciation. The Japanese Small Company Portfolio is a Feeder Portfolio and pursues its objective by investing substantially all of its assets in its corresponding Master Fund, The Japanese Small Company Series (the “Japanese Small Company Series”) of The DFA Investment Trust Company (the “Trust”), which has the same investment objective and policies as the Portfolio.
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 Japanese Small Company 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 Japanese Small Company Series 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 Japanese Small Company Portfolio’s performance. During the most recent fiscal year, the Japanese Small Company Series’ portfolio turnover rate was 7% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 7.00%
Expenses Represent Both Master and Feeder [Text] rr_ExpensesRepresentBothMasterAndFeeder The “Management Fee” includes an investment management fee payable by the Japanese Small Company Series and an administration fee payable by the Japanese Small Company Portfolio. The amounts set forth under “Other Expenses” and “Total Annual Fund Operating Expenses” reflect the direct expenses of the Japanese Small Company Portfolio and the indirect expenses of the Portfolio’s portion of the expenses of the Japanese Small Company Series.
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 Japanese Small Company 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:
Expense Example Closing [Text Block] rr_ExpenseExampleClosingTextBlock The Example reflects the aggregate estimated annual operating expenses of the Japanese Small Company Portfolio and the Japanese Small Company Portfolio’s portion of the expenses of the Japanese Small Company Series.
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock Dimensional Fund Advisors LP (the "Advisor") believes that equity investing should involve a long-term view and a systematic focus on sources of expected returns, not on stock picking or market timing. In constructing an investment portfolio, the Advisor identifies a broadly diversified universe of eligible securities with precisely-defined risk and return characteristics. It then places priority on efficiently managing portfolio turnover and keeping trading costs low. In general, the Advisor does not intend to purchase or sell securities for the investment portfolio based on prospects for the economy, the securities markets or the individual issuers whose shares are eligible for purchase.

The Japanese Small Company Portfolio pursues its investment objective by investing substantially all of its assets in the Japanese Small Company Series. The Japanese Small Company Series, using a market capitalization weighted approach, purchases a broad and diverse group of readily marketable securities of small companies associated with Japan. A company's market capitalization is the number of its shares outstanding times its price per share. In general, the higher the relative market capitalization of a Japanese small company, the greater its representation in the Series. The Advisor may modify market capitalization weights and even exclude companies after considering such factors as free float, momentum, trading strategies, liquidity management, and expected profitability, as well as other factors that the Advisor determines to be appropriate, given market conditions. In assessing expected profitability, the Advisor may consider different ratios, such as that of earnings or profits from operations relative to book value or assets.

As a non-fundamental policy, under normal circumstances, the Japanese Small Company Series will invest at least 80% of its net assets in securities of Japanese small companies. The Advisor first ranks eligible companies by market capitalization. The Advisor then determines the universe of eligible securities by defining the maximum market capitalization of a small company in Japan. As of December 31, 2012, the Advisor considered Japanese small companies to be those companies with a market capitalization below $1,378 million. This dollar amount will change due to market conditions.

The Japanese Small Company Series may gain exposure to companies associated with Japan by purchasing equity securities in the form of depositary receipts, which may be listed or traded outside the issuer's domicile country. The Japanese Small Company Series and the Japanese Small Company Portfolio each may use derivatives, such as futures contracts and options on futures contracts for Japanese equity securities and indices or other equity market securities and indices, including those of the United States, to gain market exposure on their uninvested cash pending investment in securities or to maintain liquidity to pay redemptions.

The Japanese Small Company Series may lend its portfolio securities to generate additional income.
Risk [Heading] rr_RiskHeading PRINCIPAL RISKS
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock 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 Japanese Small Company Series does not hedge foreign currency risk.

Japan Market Risk: Because the Japanese Small Company Series concentrates investments in Japan, the Japanese Small Company Portfolio's performance is expected to be closely tied to the social, political and economic conditions within Japan and to be more volatile than the performance of funds with more geographically diverse investments.

Small Company Risk: Securities of small companies are often less liquid than those of large companies and this could make it difficult to sell a small company security at a desired time or price. As a result, small company stocks may fluctuate relatively more in price. In general, smaller capitalization companies are also more vulnerable than larger companies to adverse business or economic developments and they may have more limited resources.

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 Japanese Small Company Series that owns them, and, in turn, the Japanese Small Company Portfolio itself, to rise or fall. Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.

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. The use of derivatives for non-hedging purposes may be considered more speculative than other types of investments. When the Japanese Small Company Series and the Japanese Small Company Portfolio use derivatives, the Japanese Small Company Portfolio will be directly exposed to the risks of that derivative. Derivative instruments are subject to a number of risks including 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.

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 Japanese Small Company Series may lose money and there may be a delay in recovering the loaned securities. The Japanese Small Company Series 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 Japanese Small Company 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 Japanese Small Company 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 Japanese Small Company 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 Japanese Small Company 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 Japanese Small Company 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 Japanese Small Company 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 Japanese Small Company Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading Japanese Small Company Portfolio Institutional Class Shares
Total Returns (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2003-December 2012

Highest Quarter    Lowest Quarter
26.51 (1/04-3/04)    -15.28 (1/09-3/09)
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 Japanese Small Company 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.
JAPANESE SMALL COMPANY PORTFOLIO | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.50% [1]
Other Expenses rr_OtherExpensesOverAssets 0.07% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.57% [1]
1 Year rr_ExpenseExampleYear01 58
3 Years rr_ExpenseExampleYear03 183
5 Years rr_ExpenseExampleYear05 318
10 Years rr_ExpenseExampleYear10 714
2003 rr_AnnualReturn2003 55.79%
2004 rr_AnnualReturn2004 30.41%
2005 rr_AnnualReturn2005 38.29%
2006 rr_AnnualReturn2006 (12.07%)
2007 rr_AnnualReturn2007 (8.53%)
2008 rr_AnnualReturn2008 (12.12%)
2009 rr_AnnualReturn2009 3.21%
2010 rr_AnnualReturn2010 17.50%
2011 rr_AnnualReturn2011 (1.01%)
2012 rr_AnnualReturn2012 4.16%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2004
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 26.51%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2009
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (15.28%)
One Year rr_AverageAnnualReturnYear01 4.16%
Five Years rr_AverageAnnualReturnYear05 1.90%
Ten Years rr_AverageAnnualReturnYear10 9.52%
JAPANESE SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 3.92%
Five Years rr_AverageAnnualReturnYear05 1.64%
Ten Years rr_AverageAnnualReturnYear10 9.28%
JAPANESE SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 3.12%
Five Years rr_AverageAnnualReturnYear05 1.61%
Ten Years rr_AverageAnnualReturnYear10 8.48%
JAPANESE SMALL COMPANY PORTFOLIO | MSCI Japan Small Cap Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 4.01%
Five Years rr_AverageAnnualReturnYear05 (0.13%)
Ten Years rr_AverageAnnualReturnYear10 7.69%
[1] The "Management Fee" includes an investment management fee payable by the Japanese Small Company Series and an administration fee payable by the Japanese Small Company Portfolio. The amounts set forth under "Other Expenses" and "Total Annual Fund Operating Expenses" reflect the direct expenses of the Japanese Small Company Portfolio and the indirect expenses of the Portfolio's portion of the expenses of the Japanese Small Company Series.