Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	T. Rowe Price Index Trust, Inc.
Prospectus Date	rr_ProspectusDate	Nov. 20, 2015
Investor Class | T. Rowe Price Mid-Cap Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	T. Rowe Price Mid-Cap 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 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.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Fees and Expenses of the Fund 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	April 30, 2018
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. A portfolio turnover rate is not shown since the fund had not commenced operations during its most recent fiscal year.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	Other expenses are estimated for the current fiscal year.
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, the fund’s operating expenses remain the same, and the expense limitation currently in place is not renewed. The figures have been adjusted to reflect fee waivers or expense reimbursements only in the periods for which the expense limitation arrangement is expected to continue. 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 track the investment return of mid-capitalization U.S. stocks by seeking to match the performance of the Russell Select Midcap Index (“benchmark index”). The fund seeks to employ a full replication strategy, which involves investing substantially all of its assets in all of the stocks in the benchmark index and seeking to maintain holdings of each stock in proportion to its weight in the index. Under normal conditions, the fund invests at least 80% of its net assets (including any borrowings for investment purposes) in mid-cap stocks and in stocks that are included in its benchmark index. The fund defines mid-cap stocks as those whose market capitalization, at the time of purchase, falls within the range of its benchmark index. As of September 30, 2015, the market capitalization range for the benchmark index was approximately $489 million to $33 billion. T. Rowe Price compares the composition of the fund to that of the benchmark index. If a material misweighting develops, the portfolio manager seeks to rebalance the portfolio in an effort to realign the fund with the benchmark index. The Russell Select Midcap Index, which is constructed by the Russell Investment Group, is a mid-cap stock index designed to measure the performance of the mid-cap segment of the U.S. equity universe. It includes approximately 500 mid-cap U.S. securities based on market capitalization without consideration to a growth or value style. The benchmark index is generally constructed by including all of the stocks in the Russell 1000 Index except for those also represented in the S&P 500 Index, and is reconstituted periodically to ensure larger or smaller stocks do not distort the performance and characteristics of the mid-cap opportunity set. The benchmark index and the fund may at times have significant exposure to financial and information technology companies. However, the stock components of the benchmark index, and the degree to which these stocks represent specific industries or sectors, may change over time. While most assets will be invested in common stocks, the fund may also purchase stock index futures contracts. Futures 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, or as an efficient means of gaining mid-cap stock exposure. 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 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 specific 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. Mid-cap stock risk Investing primarily in issuers within the same market capitalization category carries the risk that the category may be out of favor due to current market conditions or investor sentiment. Because the fund invests primarily in securities issued by mid-cap companies, it is likely to be more volatile than a fund that focuses on securities issued by large companies. Medium-sized companies typically have less seasoned management, narrower product lines, and less capital reserves and liquidity than larger companies, and are therefore more sensitive to economic, market, and industry changes. 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. The risk of tracking error is increased to the extent the fund is unable to fully replicate its benchmark index, which could result from changes in the composition of the index or the timing of purchases and redemptions of fund shares. Industry risk To the extent the benchmark index and the fund have significant exposure to specific industries or sectors, the fund may be more susceptible to developments affecting those industries and sectors. For example, the fund may at times have significant investments in financial and information technology companies, which could result in the fund performing poorly during a downturn in one or more of the industries that heavily impact financial or information technology companies. The profitability of financial companies can be adversely affected by, among other things, regulatory changes, the availability of capital and cost to borrow, the rate of debt defaults, interest rates, and price competition. Information technology companies face intense competition, may have limited financial resources, personnel or product lines, their products may face obsolescence due to rapid technological developments, and the loss or impairment of their patent and intellectual property rights may adversely affect their profitability. 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 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.
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	Because the fund commenced operations in 2015, there is no historical performance information shown here. Performance history will be presented after the fund has been in operation for one full calendar year. Current performance information may be obtained by calling 1-800-638-8790.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Because the fund commenced operations in 2015, there is no historical performance information shown here. Performance history will be presented after the fund has been in operation for one full calendar year.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-638-8790
Investor Class | T. Rowe Price Mid-Cap Index Fund | T. Rowe Price Mid-Cap Index Fund
Risk/Return:	rr_RiskReturnAbstract
Management fees	rr_ManagementFeesOverAssets	0.12%
Other expenses	rr_OtherExpensesOverAssets	5.38%	[1]
Total annual fund operating expenses	rr_ExpensesOverAssets	5.50%
Fee waiver/expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(5.18%)	[2]
Total annual fund operating expenses after fee waiver/expense reimbursement	rr_NetExpensesOverAssets	0.32%	[2]
1 year	rr_ExpenseExampleYear01	$ 33
3 years	rr_ExpenseExampleYear03	$ 451

[1] Other expenses are estimated for the current fiscal year.

[2] T. Rowe Price Associates, Inc. has agreed (through April 30, 2018) to waive its fees and/or bear any expenses (excluding interest, expenses related to borrowings, taxes and brokerage, extraordinary expenses and acquired fund fees) that would cause the fund’s ratio of expenses to average daily net assets to exceed 0.32%. Termination of the agreement would require approval by the fund’s Board of Directors. Fees waived and expenses paid under this agreement are subject to reimbursement to T. Rowe Price Associates, Inc. by the fund whenever the fund’s expense ratio is below 0.32%. However, no reimbursement will be made more than three years after the waiver or payment, or if it would result in the expense ratio exceeding 0.32% (excluding interest, expenses related to borrowings, taxes and brokerage, extraordinary expenses and acquired fund fees).