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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName T. Rowe Price Index Trust, Inc.
Prospectus Date rr_ProspectusDate May 01, 2016
T. Rowe Price Extended Equity Market Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading T. Rowe Price

Extended Equity Market Index Fund

SUMMARY
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The fund seeks to track the performance of a benchmark index that measures the investment return of small- and mid-capitalization U.S. stocks.
Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses that you may pay if you buy and hold shares of the fund.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Fees and Expenses of the Fund

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 fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when 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 fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 23.0% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 23.00%
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent Restated to reflect current fees.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the fund 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 fund’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 Investments, Risks, and Performance

Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The fund attempts to match the investment return of small- and mid-capitalization U.S. stocks by seeking to match the performance of its benchmark index, the S&P Completion Index (“S&P Index”). The S&P Index consists of primarily small- and mid-capitalization stocks and generally includes those U.S. stocks that are not included in the Standard & Poor’s 500 Stock Index.

The fund uses a sampling strategy, investing substantially all of its assets in a group of stocks representative of the sector allocations, financial characteristics, and other attributes of the S&P Index. The fund does not attempt to fully replicate the index by owning each of the stocks in the index. The S&P Index includes approximately 3,400 stocks as of February 29, 2016.

In an attempt to track the S&P Index, we select stocks based on industry, size, and other characteristics. For example, if technology stocks made up 15% of the S&P Index, the fund would invest about 15% of its assets in technology stocks with similar characteristics. Several factors are considered in selecting representative stocks, including historical price movement, market capitalization, transaction costs, and others.

Under normal conditions, the fund will invest at least 80% of its net assets (including any borrowings for investment purposes) in stocks that are included in its benchmark index. T. Rowe Price compares the composition of the fund to that of the index. If a material misweighting develops, the portfolio managers seek to rebalance the portfolio in an effort to realign it with its index.

While most assets will be invested in common stocks, the fund may also purchase stock index futures contracts and exchange-traded funds. Futures and exchange-traded funds would typically be used to reduce cash balances in the fund and increase the level of fund assets exposed to common stocks represented in the fund’s benchmark index. In addition, the fund lends its portfolio securities as a means of generating additional income.

While there is no guarantee, the correlation between the fund and its benchmark index is expected to be at least 0.95. A correlation of 1.00 indicates that the returns of the fund and the index will always move in the same direction (but not necessarily by the same amount). A correlation of 0.00 would mean price movements in the fund are unrelated to price movements in the index.

The fund may sell securities to better align its portfolio with the characteristics of its benchmark index or to satisfy redemption requests. The fund will generally seek to sell securities that have been removed from the benchmark index within a reasonable timeframe taking into consideration market conditions.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock As with any mutual fund, there is no guarantee that the fund will achieve its objective. The fund’s share price fluctuates, which means you could lose money by investing in the fund. The principal risks of investing in this fund are summarized as follows:

Risks of U.S. stock investing Stocks generally fluctuate in value more than bonds and may decline significantly over short time periods. There is a chance that stock prices overall will decline because stock markets tend to move in cycles, with periods of rising and falling prices. The value of a stock in which the fund invests may decline due to general weakness in the U.S. stock market, such as when the U.S. financial markets decline, or because of factors that affect a particular company or industry.

Small- and mid-cap stock risk Because the fund invests primarily in small- and medium-sized companies, its share price could be more volatile than a fund that is exposed to only large companies. Small- and medium-sized companies often have less experienced management, narrower product lines, more limited financial resources, and less publicly available information than larger companies. Smaller companies may have limited trading markets and tend to be more sensitive to changes in overall economic conditions.

Index investing risk Because the fund is passively managed and seeks to match the performance of its benchmark index, holdings are generally not reallocated based on changes in market conditions or outlook for a specific security, industry, or market sector. As a result, the fund’s performance may lag the performance of actively managed funds.

Tracking error The returns of the fund are expected to be slightly below the returns of its benchmark index (referred to as “tracking error”) because the fund incurs fees and transaction expenses while the index has no fees or expenses. Increased tracking error could also result from changes in the composition of the index or the timing of purchases and redemptions of fund shares. The fund uses a sampling strategy and does not attempt to fully replicate its benchmark index, which increases the potential for the fund’s performance to deviate from that of its index.

Futures risk The fund’s use of stock index futures exposes it to potential volatility and losses in excess of direct investments in the contract’s underlying assets. The values of the fund’s positions in stock index futures tend to fluctuate in response to changes in the value of the underlying index, which exposes the fund to the risk that the underlying index will not move in a direction that is favorable to the fund. While the value of a stock index futures contract tends to correlate with the value of the underlying index, differences between the futures market and the value of the underlying index may result in an imperfect correlation. Since losses could result from market movement, the fund may need to sell other portfolio securities at disadvantageous times in order to meet daily margin requirements. The futures markets may experience reduced liquidity, which could result in losses to the fund and cause the fund to be unable to settle its futures positions.

Exchange-traded fund risk To the extent the fund invests in exchange-traded funds, the fund will bear its proportionate share of each exchange-traded fund’s fees and expenses. An investment in an exchange-traded fund involves substantially the same risks as investing directly in the exchange-traded fund’s underlying assets, although an exchange-traded fund may trade at a premium or discount to the actual net asset value of its portfolio securities, may have greater price volatility than its underlying assets, and its shares may be less liquid.

Securities lending risk Securities lending involves the risk that the fund may lose money because the borrower of the loaned securities fails to return the securities to the fund in a timely manner or not at all. The fund may also lose money if there is a decline in the value of the collateral provided for loaned securities or a decline in the value of any investments made with cash collateral. In addition, securities lending activities may cause adverse tax consequences for the fund.
Risk Lose Money [Text] rr_RiskLoseMoney The fund’s share price fluctuates, which means you could lose money by investing in the fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart showing calendar year returns and the average annual total returns table provide some indication of the risks of investing in the fund by showing how much returns can differ from year to year and how the fund’s average annual returns for certain periods compare with the returns of a relevant broad-based market index, as well as with the returns of one or more comparative indexes that have investment characteristics similar to those of the fund. The fund’s performance information represents only past performance (before and after taxes) and is not necessarily an indication of future results.

The fund can also experience short-term performance swings, as shown by the best and worst calendar quarter returns during the years depicted.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart showing calendar year returns and the average annual total returns table provide some indication of the risks of investing in the fund by showing how much returns can differ from year to year and how the fund’s average annual returns for certain periods compare with the returns of a relevant broad-based market index, as well as with the returns of one or more comparative indexes that have investment characteristics similar to those of the fund.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-225-5132
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress troweprice.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The fund’s performance information represents only past performance (before and after taxes) and is not necessarily an indication of future results.
Bar Chart [Heading] rr_BarChartHeading Extended Equity Market Index Fund
Calendar Year Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
  Quarter
 Ended		  Total
Return
Best Quarter 6/30/09 21.55%
Worst Quarter 12/31/08 -26.52%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns

Periods ended
December 31, 2015
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns 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 Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or individual retirement account.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher In some cases, the figure shown for “returns after taxes on distributions and sale of fund shares” may be higher than the figure shown for “returns before taxes” because the calculations assume the investor received a tax deduction for any loss incurred on the sale of shares.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock In addition, the average annual total returns table shows hypothetical after-tax returns to demonstrate how taxes paid by a shareholder may influence returns. After-tax returns 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. After-tax returns shown are not relevant to investors who hold their fund shares through tax-deferred arrangements, such as a 401(k) account or individual retirement account. In some cases, the figure shown for “returns after taxes on distributions and sale of fund shares” may be higher than the figure shown for “returns before taxes” because the calculations assume the investor received a tax deduction for any loss incurred on the sale of shares.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock Updated performance information is available through troweprice.com or may be obtained by calling 1-800-225-5132.
T. Rowe Price Extended Equity Market Index Fund | T. Rowe Price Extended Equity Market Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) rr_MaximumDeferredSalesChargeOverOther none
Redemption fee (as a percentage of amount redeemed on shares held for 90 days or less) rr_RedemptionFeeOverRedemption 0.50%
Maximum account fee rr_MaximumAccountFee $ 20 [1]
Management fees rr_ManagementFeesOverAssets 0.35% [2]
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets none
Total annual fund operating expenses rr_ExpensesOverAssets 0.35% [2]
1 year rr_ExpenseExampleYear01 $ 36
3 years rr_ExpenseExampleYear03 113
5 years rr_ExpenseExampleYear05 197
10 years rr_ExpenseExampleYear10 $ 443
2006 rr_AnnualReturn2006 15.76%
2007 rr_AnnualReturn2007 4.07%
2008 rr_AnnualReturn2008 (38.50%)
2009 rr_AnnualReturn2009 36.69%
2010 rr_AnnualReturn2010 27.45%
2011 rr_AnnualReturn2011 (3.61%)
2012 rr_AnnualReturn2012 18.97%
2013 rr_AnnualReturn2013 38.37%
2014 rr_AnnualReturn2014 7.59%
2015 rr_AnnualReturn2015 (3.29%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 21.55%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (26.52%)
1 Year rr_AverageAnnualReturnYear01 (3.29%)
5 Years rr_AverageAnnualReturnYear05 10.55%
10 Years rr_AverageAnnualReturnYear10 7.86%
T. Rowe Price Extended Equity Market Index Fund | Returns after taxes on distributions | T. Rowe Price Extended Equity Market Index Fund  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (4.41%)
5 Years rr_AverageAnnualReturnYear05 9.71%
10 Years rr_AverageAnnualReturnYear10 7.17%
T. Rowe Price Extended Equity Market Index Fund | Returns after taxes on distributions and sale of fund shares | T. Rowe Price Extended Equity Market Index Fund  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (1.05%)
5 Years rr_AverageAnnualReturnYear05 8.28%
10 Years rr_AverageAnnualReturnYear10 6.27%
T. Rowe Price Extended Equity Market Index Fund | S&P Completion Index (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (3.35%)
5 Years rr_AverageAnnualReturnYear05 10.37%
10 Years rr_AverageAnnualReturnYear10 7.68%
T. Rowe Price Extended Equity Market Index Fund | Combined Index Portfolio (reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 (3.35%) [3]
5 Years rr_AverageAnnualReturnYear05 10.37% [3]
10 Years rr_AverageAnnualReturnYear10 7.81% [3]
[1] Subject to certain exceptions, accounts with a balance of less than $10,000 are charged an annual $20 fee.
[2] Restated to reflect current fees.
[3] Combined Index Portfolio is a blended benchmark composed of 100% Dow Jones Wilshire 4500 Completion Index (1/30/98 - 3/31/07), then 100% S&P Completion Index (from 4/1/07 forward). Indices and percentages may vary over time.