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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2018
World Core Equity Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading World Core Equity Portfolio
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the World Core Equity Portfolio (formerly, Dimensional Retirement Equity Fund II) (the “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 World Core Equity 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, 2019
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. The World Core Equity Portfolio does not pay transaction costs when buying and selling shares of other mutual funds (the “Underlying Funds”); however, the Underlying Funds pay transaction costs when buying and selling securities for their portfolios. The transaction costs incurred by the Underlying Funds, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Portfolio’s performance. During the most recent fiscal year, the World Core Equity Portfolio’s portfolio turnover rate was 4% based on the weighted average portfolio turnover ratios of each of the Portfolio’s underlying investments.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 4.00%
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 World Core Equity 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 World Core Equity Portfolio is a “fund of funds,” which means that the World Core Equity Portfolio generally allocates its assets among other mutual funds managed by the Advisor although it also has the ability to invest directly in securities. The World Core Equity Portfolio seeks to achieve exposure to a broad portfolio of securities of both U.S. companies and non-U.S. companies associated with countries with developed and emerging markets, which may include frontier markets (emerging markets in an earlier stage of development), by primarily purchasing shares of U.S. Core Equity 1 Portfolio, U.S. Large Company Portfolio, U.S. Large Cap Equity Portfolio, International Core Equity Portfolio, Large Cap International Portfolio, and Emerging Markets Core Equity Portfolio (the “Underlying Funds”). The World Core Equity Portfolio may have exposure to companies in all the market capitalization ranges.

The World Core Equity Portfolio typically allocates its investments among the Underlying Funds in the following manner: 35% to 80% in the U.S. Core Equity 1 Portfolio, U.S. Large Cap Equity Portfolio and/or U.S. Large Company Portfolio; 15% to 55% in the International Core Equity Portfolio and/or Large Cap International Portfolio; and 5% to 20% in the Emerging Markets Core Equity Portfolio. Allocations by the World Core Equity Portfolio among the Underlying Funds within the ranges described above are determined by the relative value of the holdings of the Underlying Funds. Periodically, the Advisor will review the allocations for the World Core Equity Portfolio in each Underlying Fund and may adjust allocations to the Underlying Funds or may add or remove Underlying Funds in the Portfolio without notice to shareholders.

As a non-fundamental policy, under normal circumstances, at least 80% of the World Core Equity Portfolio’s net assets will be invested directly, or indirectly through its investment in the Underlying Funds, in equity securities.

The World Core Equity Portfolio and each Underlying Fund may purchase or sell futures contracts and options on futures contracts for equity securities and indices, to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio or Underlying Fund. The World Core Equity Portfolio and Underlying Funds 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 World Core Equity Portfolio and each Underlying Fund may invest in exchange-traded funds (ETFs) and similarly structured pooled investments for the purposes of gaining exposure to the equity markets, while maintaining liquidity. In addition to money market instruments and other short-term investments, the World Core Equity Portfolio and each Underlying Fund may invest in affiliated and unaffiliated registered and unregistered money market funds. The World Core Equity Portfolio and Underlying Funds may invest in such money market funds and other short-term investments to manage cash pending investment in other securities or to maintain liquidity for the payment of redemptions or other purposes. Investments in ETFs and money market funds may involve a duplication of certain fees and expenses.

The World Core Equity Portfolio and Underlying Funds may lend their portfolio securities to generate additional income.

A summary of the investment strategies and policies of the Underlying Funds in which the World Core Equity Portfolio invests as of the date of this Prospectus is described in the Portfolio’s Prospectus in the section entitled “ADDITIONAL INFORMATION ON INVESTMENT OBJECTIVES AND POLICIES”.
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.

Fund of Funds Risk: The investment performance of the World Core Equity Portfolio is affected by the investment performance of the Underlying Funds in which the Portfolio invests. The ability of the Portfolio to achieve its investment objective depends on the ability of the Underlying Funds to meet their investment objectives and on the Advisor’s decisions regarding the allocation of the Portfolio’s assets among the Underlying Funds. The Portfolio may allocate assets to an Underlying Fund or asset class that underperforms other funds or asset classes. There can be no assurance that the investment objective of the Portfolio or any Underlying Fund will be achieved. When the Portfolio invests in Underlying Funds, investors are exposed to a proportionate share of the expenses of those Underlying Funds in addition to the expenses of the Portfolio. Through its investments in the Underlying Funds, the Portfolio is subject to the risks of the Underlying Funds’ investments. The risks of the World Core Equity Portfolio’s and Underlying Funds’ investments are described below.

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 or Underlying Fund 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 Underlying Funds do 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.

Value Investment Risk: Value stocks may perform differently from the market as a whole and following a value-oriented investment strategy may cause the World Core Equity Portfolio and the Underlying Funds to at times underperform equity funds that use other investment strategies.

Emerging Markets Risk: Numerous emerging market countries have a history of, and continue to experience serious, and potentially continuing, economic and political problems. Stock markets in many emerging market countries are relatively small, expensive to trade in and generally have higher risks than those in developed markets. Securities in emerging markets also may be less liquid than those in developed markets and foreigners are often limited in their ability to invest in, and withdraw assets from, these markets. Additional restrictions may be imposed under other conditions. Frontier market countries generally have smaller economies or less developed capital markets and, as a result, the risks of investing in emerging market countries are magnified in frontier market countries.

Derivatives Risk: Derivatives are instruments, such as swaps, 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 World Core Equity Portfolio or an Underlying Fund uses derivatives, the Portfolio or Underlying Fund 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 or Underlying Fund 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, an Underlying Fund may lose money and there may be a delay in recovering the loaned securities. An Underlying Fund 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. To the extent that the Portfolio holds securities directly and lends those securities, it will be also subject to the foregoing risks with respect to its loaned securities.

Cyber Security Risk: The World Core Equity 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 World Core Equity 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, five 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 World Core Equity Portfolio’s 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 World Core Equity 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 World Core Equity Portfolio Institutional Class Shares—Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2013-December 2017

Highest Quarter    Lowest Quarter
9.34% (10/13–12/13)
    -9.91% (7/15–9/15)
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 World Core Equity Portfolio’s 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.
World Core Equity Portfolio | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.30%
Other Expenses rr_OtherExpensesOverAssets 0.02%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.28%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.60%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.25% [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.35%
1 Year rr_ExpenseExampleYear01 $ 36
3 Years rr_ExpenseExampleYear03 167
5 Years rr_ExpenseExampleYear05 310
10 Years rr_ExpenseExampleYear10 726
1 Year rr_ExpenseExampleNoRedemptionYear01 36
3 Years rr_ExpenseExampleNoRedemptionYear03 167
5 Years rr_ExpenseExampleNoRedemptionYear05 310
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 726
2013 rr_AnnualReturn2013 29.52%
2014 rr_AnnualReturn2014 2.48%
2015 rr_AnnualReturn2015 (2.46%)
2016 rr_AnnualReturn2016 10.72%
2017 rr_AnnualReturn2017 25.09%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2013
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 9.34%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2015
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (9.91%)
1 Year rr_AverageAnnualReturnYear01 25.09%
5 Years rr_AverageAnnualReturnYear05 12.39%
Since Inception rr_AverageAnnualReturnSinceInception 11.75%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 07, 2012
World Core Equity Portfolio | Return After Taxes on Distributions | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 24.37%
5 Years rr_AverageAnnualReturnYear05 11.66%
Since Inception rr_AverageAnnualReturnSinceInception 11.06%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 07, 2012
World Core Equity Portfolio | Return After Taxes on Distributions and Sale of Portfolio Shares | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 14.61%
5 Years rr_AverageAnnualReturnYear05 9.71%
Since Inception rr_AverageAnnualReturnSinceInception 9.27%
Inception Date rr_AverageAnnualReturnInceptionDate Mar. 07, 2012
World Core Equity Portfolio | MSCI All Country World Index (net dividends) (reflects no deduction for fees, expenses, or taxes on sales)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 23.97%
5 Years rr_AverageAnnualReturnYear05 10.80%
Since Inception rr_AverageAnnualReturnSinceInception 10.49%
[1] Dimensional Fund Advisors LP (the "Advisor") has agreed to waive certain fees and in certain instances, assume certain expenses of the World Core Equity Portfolio. The Amended and Restated Fee Waiver and Expense Assumption Agreement for the Portfolio will remain in effect through February 28, 2019, 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.