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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName DFA INVESTMENT DIMENSIONS GROUP INC
Prospectus Date rr_ProspectusDate Feb. 28, 2015
ENHANCED U.S. LARGE COMPANY PORTFOLIO  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Enhanced U.S. Large Company Portfolio
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Enhanced U.S. Large Company Portfolio seeks, as its investment objective, to achieve a total return which exceeds the total return performance of the S&P 500® Index. Total return comprises income and 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 Enhanced U.S. Large 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 Enhanced U.S. Large Company 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 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 Portfolio’s performance. During the most recent fiscal year, the Enhanced U.S. Large Company Portfolio’s portfolio turnover rate was 202% of the average value of its investment portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 202.00%
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 Enhanced U.S. Large 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:
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Enhanced U.S. Large Company Portfolio seeks to outperform the S&P 500® Index primarily through investment in S&P 500® Index futures and short-term fixed income obligations. The Enhanced U.S. Large Company Portfolio may invest in all of the stocks represented in the S&P 500® Index, options on stock indices, stock index futures, options on stock index futures, swap agreements on stock indices and shares of investment companies, such as exchange-traded funds (ETFs) that invest in stocks represented in the S&P 500® Index or other similar stock indices. The Portfolio invests in fixed income obligations, which may include securities of foreign issuers. The Portfolio may, from time to time, also invest in options on stock indices, stock index futures, options on stock index futures and swap agreements based on indices other than, but similar to, the S&P 500® Index (such instruments whether or not based on the S&P 500® Index are hereinafter collectively referred to as “Index Derivatives”). The S&P 500® Index comprises a broad and diverse group of stocks. Generally, these are the U.S. stocks with the largest market capitalizations and, as a group, they represent approximately 75% of the total market capitalization of all publicly traded U.S. stocks. Dimensional Fund Advisors LP (the “Advisor”) considers stocks that comprise the S&P 500® Index to be those of large companies. Under normal circumstances, the Enhanced U.S. Large Company Portfolio will invest at least 80% of its net assets in short-term fixed income obligations that are overlaid by futures, swaps and other derivatives of the S&P 500® Index to create exposure to the performance of large U.S. companies or in securities of large U.S. companies directly. Alternatively, the Portfolio may invest at least 80% of its net assets directly in securities of large companies.

The Enhanced U.S. Large Company Portfolio may invest all of its assets in Index Derivatives. Certain of these Index Derivatives may be considered speculative and may subject the Portfolio to additional risks. Assets of the Portfolio not invested in S&P 500® Index or Index Derivatives may be invested in short-term fixed income obligations including: U.S. government obligations, U.S. government agency obligations, corporate debt obligations, bank obligations, commercial paper, repurchase agreements, foreign government and agency obligations, supranational organization obligations, foreign issuer obligations and eurodollar obligations. The Portfolio’s investment in fixed income obligations will be considered investment grade at the time of purchase. The fixed income obligations purchased by the Portfolio will typically mature within two years from the date of settlement and the average dollar-weighted maturity of the fixed income obligations will be two years or less.

The Enhanced U.S. Large Company Portfolio may use foreign currency contracts to hedge foreign currency risks or to transfer balances from one currency to another. The Enhanced U.S. Large Company Portfolio uses index swap agreements and/or stock index futures to hedge against changes in securities prices. The Enhanced U.S. Large Company Portfolio may use derivatives, such as futures contracts and options on futures contracts, to adjust market exposure based on actual or expected cash inflows to or outflows from the Portfolio. Additionally, the Enhanced U.S. Large Company Portfolio uses index swap agreements and stock index futures to attempt to achieve its investment objectives.

The Enhanced U.S. Large Company Portfolio may lend its portfolio securities to generate additional income.

About the S&P 500® Index: The Standard & Poor’s 500 Composite Stock Price Index® is market capitalization weighted (adjusted for free float). Its performance is usually cyclical because it reflects periods when stock prices generally rise or fall. For information concerning Standard & Poor’s Rating Group, a division of The McGraw Hill Companies (“S&P”), and disclaimers of S&P with respect to the Enhanced U.S. Large Company Portfolio, see “Standard & Poor’s—Information and Disclaimers.”
Risk [Heading] rr_RiskHeading PRINCIPAL RISKS
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock 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 Enhanced U.S. Large Company Portfolio that owns them, to rise or fall. Because the value of your investment in the Portfolio will fluctuate, there is the risk that you will lose money.

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 Enhanced U.S. Large Company Portfolio may hedge foreign currency risk.

Foreign Government Debt Risk: The risk that: (a) the governmental entity that controls the repayment of government debt may not be willing or able to repay the principal and/or to pay the interest when it becomes due, due to factors such as political considerations, the relative size of the governmental entity’s debt position in relation to the economy, cash flow problems, insufficient foreign currency reserves, the failure to put in place economic reforms required by the International Monetary Fund or other multilateral agencies, and/or other national economic factors; (b) governments may default on their debt securities, which may require holders of such securities to participate in debt rescheduling; and (c) there is no legal or bankruptcy process by which defaulted government debt may be collected in whole or in part.

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. Derivatives can be used for hedging (attempting to reduce risk by offsetting one investment position with another) or non-hedging purposes. Hedging with derivatives may increase expenses, and there is no guarantee that a hedging strategy will work. While hedging can reduce or eliminate losses, it can also reduce or eliminate gains or cause losses if the market moves in a manner different from that anticipated by the Portfolio or if the cost of the derivative outweighs the benefit of the hedge. The use of derivatives for non-hedging purposes may be considered more speculative than other types of investments. When the Enhanced U.S. Large Company Portfolio uses derivatives, the Portfolio will be directly exposed to the risks of that derivative. 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 Enhanced U.S. Large Company Portfolio may lose money and there may be a delay in recovering the loaned securities. The Enhanced U.S. Large Company 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.

Interest Rate Risk: Fixed income securities are subject to interest rate risk because the prices of fixed income securities tend to move in the opposite direction of interest rates. When interest rates rise, fixed income security prices fall. When interest rates fall, fixed income security prices rise. In general, fixed income securities with longer maturities are more sensitive to changes in interest rates.

Credit Risk: Credit risk is the risk that the issuer of a security may be unable to make interest payments and/or repay principal when due. A downgrade to an issuer’s credit rating or a perceived change in an issuer’s financial strength may affect a security’s value, and thus, impact the Enhanced U.S. Large Company Portfolio’s performance. Below investment grade fixed income securities may also fluctuate in value more than higher quality fixed income securities and, during periods of market volatility, may be more difficult to sell at the time and price the Enhanced U.S. Large Company Portfolio desires. Credit risk is greater for fixed income securities with ratings below investment grade (BB or below by Standard & Poor’s Rating Group or Ba or below by Moody’s Investors Service, Inc.). Fixed income securities that are below investment grade involve high credit risk and are considered speculative. Government agency obligations have different levels of credit support and, therefore, different degrees of credit risk. Securities issued by agencies and instrumentalities of the U.S. Government that are supported by the full faith and credit of the United States, such as the Federal Housing Administration and Ginnie Mae, present little credit risk. Other securities issued by agencies and instrumentalities sponsored by the U.S. Government, that are supported only by the issuer’s right to borrow from the U.S. Treasury, subject to certain limitations, and securities issued by agencies and instrumentalities sponsored by the U.S. Government that are sponsored by the credit of the issuing agencies, such as Freddie Mac and Fannie Mae, are subject to a greater degree of credit risk. U.S. government agency securities issued or guaranteed by the credit of the agency may still involve a risk of non-payment of principal and/or interest.

Income Risk: Income risk is the risk that falling interest rates will cause the Enhanced U.S. Large Company Portfolio’s income to decline because, among other reasons, the proceeds from maturing short-term securities in its portfolio may be reinvested in lower-yielding securities.

Call Risk: Call risk is the risk that during periods of falling interest rates, a bond issuer will call or repay a higher-yielding bond before its maturity date, forcing the Portfolio to reinvest in bonds with lower interest rates than the original obligations.

Liquidity Risk: Liquidity risk exists when particular portfolio investments are difficult to purchase or sell. To the extent that the Enhanced U.S. Large Company Portfolio holds illiquid investments, the Portfolio’s performance may be reduced due to an inability to sell the investments at opportune prices or times. Liquid portfolio investments may become illiquid or less liquid after purchase by the Enhanced U.S. Large Company Portfolio due to low trading volume, adverse investor perceptions and/or other market developments. Liquidity risk includes the risk that the Enhanced U.S. Large Company Portfolio will experience significant net redemptions at a time when it cannot find willing buyers for its portfolio securities or can only sell its portfolio securities at a material loss. Liquidity risk can be more pronounced in periods of market turmoil.

Cyber Security Risk: The Enhanced U.S. Large 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.
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 Enhanced U.S. Large 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 the 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. Enhanced U.S. Large 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 Enhanced U.S. Large 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 Enhanced U.S. Large 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 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 Enhanced U.S. Large Company Portfolio’s past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading Enhanced U.S. Large Company Portfolio Institutional Class Shares
Total Returns (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock January 2005-December 2014

Highest Quarter    Lowest Quarter
16.84 (4/09-6/09)    -21.78 (10/08-12/08)
Performance Table Heading rr_PerformanceTableHeading Periods ending December 31, 2014
Annualized Returns (%)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns presented in the table for the Enhanced U.S. Large 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.
ENHANCED U.S. LARGE COMPANY PORTFOLIO | Enhanced U.S. Large Company Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fee rr_ManagementFeesOverAssets 0.20%
Other Expenses rr_OtherExpensesOverAssets 0.03%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.23%
1 Year rr_ExpenseExampleYear01 $ 24
3 Years rr_ExpenseExampleYear03 74
5 Years rr_ExpenseExampleYear05 130
10 Years rr_ExpenseExampleYear10 $ 293
2005 rr_AnnualReturn2005 3.58%
2006 rr_AnnualReturn2006 15.03%
2007 rr_AnnualReturn2007 5.07%
2008 rr_AnnualReturn2008 (37.28%)
2009 rr_AnnualReturn2009 28.72%
2010 rr_AnnualReturn2010 15.66%
2011 rr_AnnualReturn2011 2.25%
2012 rr_AnnualReturn2012 16.96%
2013 rr_AnnualReturn2013 32.11%
2014 rr_AnnualReturn2014 13.54%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 16.84%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (21.78%)
One Year rr_AverageAnnualReturnYear01 13.54%
Five Years rr_AverageAnnualReturnYear05 15.72%
Ten Years rr_AverageAnnualReturnYear10 7.69%
ENHANCED U.S. LARGE COMPANY PORTFOLIO | Return After Taxes on Distributions | Enhanced U.S. Large Company Portfolio  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 8.85%
Five Years rr_AverageAnnualReturnYear05 14.49%
Ten Years rr_AverageAnnualReturnYear10 6.06%
ENHANCED U.S. LARGE COMPANY PORTFOLIO | Return After Taxes on Distributions and Sale of Portfolio Shares | Enhanced U.S. Large Company Portfolio  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 9.17%
Five Years rr_AverageAnnualReturnYear05 12.26%
Ten Years rr_AverageAnnualReturnYear10 5.48%
ENHANCED U.S. LARGE COMPANY PORTFOLIO | S&P 500® Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
One Year [1] rr_AverageAnnualReturnYear01 13.69%
Five Years [1] rr_AverageAnnualReturnYear05 15.45%
Ten Years [1] rr_AverageAnnualReturnYear10 7.67%
[1] Copyright© 2010 Standard & Poor's Financial Services LLC, a subsidiary of The McGraw-Hill Companies, Inc. All rights reserved.