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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2019
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 <b>Shareholder Fees (fees paid directly from your investment): </b>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <b>Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)</b>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <b>PORTFOLIO TURNOVER </b>
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 17% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 17.00%
Expenses Represent Both Master and Feeder [Text] rr_ExpensesRepresentBothMasterAndFeeder The “Management Fee” includes an investment management fee payable by the Feeder Portfolio and an investment management fee payable by the Master Fund. For any period when the Feeder Portfolio is invested in other funds managed by Dimensional Fund Advisors LP (the “Advisor”) (collectively, “Underlying Funds”), the Advisor has contractually agreed to permanently waive the Feeder Portfolio’s direct investment management fee to the extent necessary to offset the proportionate share of any Underlying Fund’s investment management fee paid by the Feeder Portfolio through its investment in such Underlying Fund. The amounts set forth under “Other Expenses” and “Total Annual Fund Operating Expenses” reflect the direct expenses of the Feeder Portfolio and the indirect expenses of the Feeder Portfolio’s portion of the expenses of the Master Fund.
Expense Example [Heading] rr_ExpenseExampleHeading <b>EXAMPLE </b>
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. 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 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 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 adjust the representation in the Series of an eligible company, or exclude a company, after considering such factors as free float, momentum, trading strategies, liquidity, value, profitability, and other factors that the Advisor determines to be appropriate, given market conditions. Securities are considered value stocks primarily because a company’s shares have a low price in relation to their book value. In assessing value, the Advisor may consider additional factors such as price to cash flow or price to earnings ratios. In assessing profitability, the Advisor may consider different ratios, such as that of earnings or profits from operations relative to book value or assets. The criteria the Advisor uses for assessing value or profitability are subject to change from time to time.

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. Based on market capitalization data as of December 31, 2018, the Advisor would consider Japanese small companies to be those companies with a market capitalization below $1,595 million. This threshold 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 purchase or sell 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 adjust market exposure based on actual or expected cash inflows to or outflows from the Series or Portfolio. The Series and Portfolio do 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 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 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.

Equity Market Risk: Even a long-term investment approach cannot guarantee a profit. Economic, market, political, and issuer-specific conditions and events will cause the value of equity securities, and the Portfolio that owns them, to rise or fall. Stock markets tend to move in cycles, with periods of rising prices and periods of falling prices.

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.

Derivatives Risk: Derivatives are instruments, such as futures, and options thereon, and foreign currency 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 to carry more risk 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 those derivatives. Derivative instruments are subject to a number of risks including counterparty, liquidity, interest rate, market, credit and management risks, as well as 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.

Cyber Security Risk: The Japanese Small Company 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 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 http://us.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-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 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 http://us.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 <b>Japanese Small Company Portfolio Institutional Class Shares—Total Returns </b>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2009-December 2018

Highest Quarter    Lowest Quarter
24.48% (4/09–6/09)
    -17.53% (10/18–12/18)
Performance Table Heading rr_PerformanceTableHeading <b>Annualized Returns (%) </b><br/>Periods ending December 31, 2018
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-advantaged 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 0.60% [1]
Other Expenses rr_OtherExpensesOverAssets 0.03% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.63% [1]
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.10% [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.53% [1]
1 Year rr_ExpenseExampleYear01 $ 54
3 Years rr_ExpenseExampleYear03 170
5 Years rr_ExpenseExampleYear05 296
10 Years rr_ExpenseExampleYear10 665
1 Year rr_ExpenseExampleNoRedemptionYear01 54
3 Years rr_ExpenseExampleNoRedemptionYear03 170
5 Years rr_ExpenseExampleNoRedemptionYear05 296
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 665
2009 rr_AnnualReturn2009 3.21%
2010 rr_AnnualReturn2010 17.50%
2011 rr_AnnualReturn2011 (1.01%)
2012 rr_AnnualReturn2012 4.16%
2013 rr_AnnualReturn2013 23.66%
2014 rr_AnnualReturn2014 (0.93%)
2015 rr_AnnualReturn2015 13.99%
2016 rr_AnnualReturn2016 9.16%
2017 rr_AnnualReturn2017 35.68%
2018 rr_AnnualReturn2018 (19.51%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 24.48%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2018
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (17.53%)
1 Year rr_AverageAnnualReturnYear01 (19.51%)
5 Years rr_AverageAnnualReturnYear05 6.13%
10 Years rr_AverageAnnualReturnYear10 7.61%
JAPANESE SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (20.72%)
5 Years rr_AverageAnnualReturnYear05 5.44%
10 Years rr_AverageAnnualReturnYear10 7.06%
JAPANESE SMALL COMPANY PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | INSTITUTIONAL CLASS  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (10.47%)
5 Years rr_AverageAnnualReturnYear05 4.81%
10 Years rr_AverageAnnualReturnYear10 6.17%
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  
1 Year rr_AverageAnnualReturnYear01 (15.99%)
5 Years rr_AverageAnnualReturnYear05 6.36%
10 Years rr_AverageAnnualReturnYear10 8.04%
[1] The “Management Fee” includes an investment management fee payable by the Feeder Portfolio and an investment management fee payable by the Master Fund. For any period when the Feeder Portfolio is invested in other funds managed by Dimensional Fund Advisors LP (the “Advisor”) (collectively, “Underlying Funds”), the Advisor has contractually agreed to permanently waive the Feeder Portfolio’s direct investment management fee to the extent necessary to offset the proportionate share of any Underlying Fund’s investment management fee paid by the Feeder Portfolio through its investment in such Underlying Fund. The amounts set forth under “Other Expenses” and “Total Annual Fund Operating Expenses” reflect the direct expenses of the Feeder Portfolio and the indirect expenses of the Feeder Portfolio’s portion of the expenses of the Master Fund.