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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2017
International Small Cap Growth Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading International Small Cap Growth Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the International Small Cap Growth 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 International Small Cap Growth 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, 2018
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock A mutual fund generally 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 mutual fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the International Small Cap Growth Portfolio’s performance. During the most recent fiscal year, the International Small Cap Growth Portfolio’s portfolio turnover rate was 29% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 29.00%
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 International Small Cap Growth 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. The costs for the Portfolio reflect the net expenses of the Portfolio that result from the contractual expense waiver in the first year only. 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 International Small Cap Growth Portfolio, using a market capitalization weighted approach, purchases securities of small, non-U.S. companies in countries with developed markets that the Advisor determines to have high profitability and valuation ratios relative to other non-U.S. small cap companies at the time of purchase. 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. The Advisor may also adjust the representation in the International Small Cap Growth Portfolio of an eligible company, or exclude a company, after considering such factors as free float, momentum, trading strategies, liquidity, profitability, and other factors that the Advisor determines to be appropriate, given market conditions. Companies are considered to have high valuation ratios primarily because a company’s shares have a high market value in relation to their book value (a “market to book ratio”) when compared with other non-U.S. small cap companies. In assessing high valuation ratio companies, the Advisor may consider additional factors such as price to cash flow or price to earnings ratios, as well as economic conditions and developments in the issuer’s industry. Securities of companies that have high valuation ratios are often considered to be growth stocks. 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 International Small Cap Growth Portfolio intends to purchase securities of non-U.S. small cap companies that are eligible based on their valuation ratio and expected profitability and are associated with developed market countries that the Advisor has designated as approved markets. As a non-fundamental policy, under normal circumstances, the International Small Cap Growth Portfolio will invest at least 80% of its net assets in securities of small companies in the particular markets in which it invests. The Advisor determines the maximum market capitalization of a small company with respect to each country in which the Portfolio invests. In the countries or regions authorized for investment, the Advisor first ranks eligible companies listed on selected exchanges based on the companies’ market capitalizations. The Advisor then determines the universe of eligible stocks by defining the maximum market capitalization of a small company that may be purchased by the Portfolio with respect to each country or region. Based on market capitalization data as of December 31, 2016, for the International Small Cap Growth Portfolio, the highest maximum market capitalization of a small company in any country in which the International Small Cap Growth Portfolio invests would be $5,561 million. This threshold will vary by country or region. For example, based on market capitalization data as of December 31, 2016, the Advisor would consider a small company in Switzerland to have a market capitalization below $4,854 million, a small company in Norway to have a market capitalization below $2,275 million, and a small company in Japan to have a market capitalization below $1,977 million. This threshold will change due to market conditions.

The International Small Cap Growth Portfolio 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 International Small Cap Growth Portfolio also may purchase or sell futures contracts and options on futures contracts for foreign and 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 International Small Cap Growth 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 Portfolio will fluctuate, there is the risk that you will lose money. 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 International Small Cap Growth 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.

Investment Strategy Risk: Securities that have high valuation ratios and high profitability may perform differently from the market as a whole and an investment strategy purchasing these securities may cause the Portfolio to at times underperform equity funds that use other investment strategies.

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 International Small Cap Growth 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 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 to carry more risk than other types of investments. When the International Small Cap Growth 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 International Small Cap Growth Portfolio may lose money and there may be a delay in recovering the loaned securities. The International Small Cap Growth 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 International Small Cap Growth 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 International Small Cap Growth 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 International Small Cap Growth 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 International Small Cap Growth 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 International Small Cap Growth Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2013-December 2016

Highest Quarter    Lowest Quarter
13.46% (7/13–9/13)
    -7.80% (7/14–9/14)
Performance Table Heading rr_PerformanceTableHeading Annualized Returns (%)
Periods ending December 31, 2016
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the International Small Cap Growth 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.
International Small Cap Growth Portfolio | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.50%
Other Expenses rr_OtherExpensesOverAssets 0.15%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.65%
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 0.55%
1 Year rr_ExpenseExampleYear01 $ 56
3 Years rr_ExpenseExampleYear03 198
5 Years rr_ExpenseExampleYear05 352
10 Years rr_ExpenseExampleYear10 801
1 Year rr_ExpenseExampleNoRedemptionYear01 56
3 Years rr_ExpenseExampleNoRedemptionYear03 198
5 Years rr_ExpenseExampleNoRedemptionYear05 352
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 801
2013 rr_AnnualReturn2013 27.60%
2014 rr_AnnualReturn2014 (4.87%)
2015 rr_AnnualReturn2015 10.47%
2016 rr_AnnualReturn2016 1.33%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2013
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 13.46%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2014
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (7.80%)
1 Year rr_AverageAnnualReturnYear01 1.33%
Since Inception rr_AverageAnnualReturnSinceInception 8.04%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 20, 2012
International Small Cap Growth Portfolio | Return After Taxes on Distributions | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 0.79%
Since Inception rr_AverageAnnualReturnSinceInception 7.38%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 20, 2012
International Small Cap Growth Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.20%
Since Inception rr_AverageAnnualReturnSinceInception 6.24%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 20, 2012
International Small Cap Growth Portfolio | MSCI World ex USA Small Cap Growth Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 0.86%
Since Inception rr_AverageAnnualReturnSinceInception 6.69%
[1] Dimensional Fund Advisors LP (the “Advisor”) has agreed to waive certain fees and in certain instances, assume certain expenses of the International Small Cap Growth Portfolio. The Amended and Restated Fee Waiver Agreement for the Portfolio will remain in effect through February 28, 2018, 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.