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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2018
VA INTERNATIONAL SMALL PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading VA International Small Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the VA International Small Portfolio is to achieve long-term 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 VA International Small 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.
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 International Small 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. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the VA International Small Portfolio’s performance. During the most recent fiscal year, the VA International Small Portfolio’s portfolio turnover rate was 12% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 12.00%
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 International Small 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 and if such fees were included, expenses would be higher. 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 VA International Small Portfolio, using a market capitalization weighted approach, purchases securities of (1) Japanese small companies; (2) United Kingdom small companies; (3) small companies organized under the laws of certain European countries; (4) small companies associated with Australia, New Zealand and Pacific Rim Asian countries; and (5) Canadian small companies. The VA International Small Portfolio also may have some exposure to small cap equity securities associated with other countries or regions. 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 small company within an eligible country, the greater its representation in the Portfolio. Dimensional Fund Advisors LP (the “Advisor”) may adjust the representation in the VA International Small Portfolio 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. The Advisor will determine the allocation of assets among the five segments and will periodically review and modify such allocation, all in its sole discretion. As a non-fundamental policy, under normal circumstances, the Portfolio will invest at least 80% of its net assets in securities of small companies. Based on market capitalization data as of December 31, 2017, the highest maximum market capitalization of a small company in any country in which the VA International Small Portfolio invests would be $6,948 million. 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.

The VA International Small Portfolio also may gain exposure to companies associated with approved markets by purchasing equity securities in the form of depositary receipts, which may be listed or traded outside the issuer’s domicile country. The VA International Small Portfolio may purchase or sell futures contracts and options on futures contracts for foreign or U.S. equity securities and indices, 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 or to use derivatives for purposes of speculation or leveraging investment returns.

The VA International Small 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 VA International Small Portfolio will fluctuate, there is the risk that you will lose money. An investment in the VA International Small 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 VA International Small 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 VA International Small Portfolio does not hedge foreign currency risk.

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 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 VA International Small 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.

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 International Small 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 VA International Small 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 VA International Small Portfolio will fluctuate, there is the risk that you will lose money.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the VA International Small 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 VA International Small 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 http://us.dimensional.com.

The after-tax returns presented in the table for the VA International Small 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 VA International Small 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 VA International Small Portfolio—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2008-December 2017

Highest Quarter    Lowest Quarter
31.25% (4/09–6/09)
    -22.08% (7/08–9/08)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2017
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the VA International Small 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.
VA INTERNATIONAL SMALL PORTFOLIO | VA International Small Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 0.50%
Other Expenses rr_OtherExpensesOverAssets 0.07%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.57%
1 Year rr_ExpenseExampleYear01 $ 58
3 Years rr_ExpenseExampleYear03 183
5 Years rr_ExpenseExampleYear05 318
10 Years rr_ExpenseExampleYear10 714
1 Year rr_ExpenseExampleNoRedemptionYear01 58
3 Years rr_ExpenseExampleNoRedemptionYear03 183
5 Years rr_ExpenseExampleNoRedemptionYear05 318
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 714
2008 rr_AnnualReturn2008 (42.88%)
2009 rr_AnnualReturn2009 39.87%
2010 rr_AnnualReturn2010 24.80%
2011 rr_AnnualReturn2011 (14.82%)
2012 rr_AnnualReturn2012 19.41%
2013 rr_AnnualReturn2013 27.07%
2014 rr_AnnualReturn2014 (5.78%)
2015 rr_AnnualReturn2015 5.81%
2016 rr_AnnualReturn2016 6.23%
2017 rr_AnnualReturn2017 29.95%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 31.25%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (22.08%)
1 Year rr_AverageAnnualReturnYear01 29.95%
5 Years rr_AverageAnnualReturnYear05 11.83%
10 Years rr_AverageAnnualReturnYear10 5.90%
VA INTERNATIONAL SMALL PORTFOLIO | Return After Taxes on Distributions | VA International Small Portfolio  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 27.70%
5 Years rr_AverageAnnualReturnYear05 10.00%
10 Years rr_AverageAnnualReturnYear10 4.46%
VA INTERNATIONAL SMALL PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | VA International Small Portfolio  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 17.47%
5 Years rr_AverageAnnualReturnYear05 8.60%
10 Years rr_AverageAnnualReturnYear10 4.11%
VA INTERNATIONAL SMALL PORTFOLIO | MSCI World ex USA Small Cap Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 31.04%
5 Years rr_AverageAnnualReturnYear05 11.37%
10 Years rr_AverageAnnualReturnYear10 5.16%