Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Feb. 23, 2012
SIIT Extended Market Index Fund (Prospectus Summary) | SIIT Extended Market Index Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return, Heading	rr_RiskReturnHeading	EXTENDED MARKET INDEX FUND Fund Summary
Investment Objective, Heading	rr_ObjectiveHeading	Investment Goal
investment Objective, Primary	rr_ObjectivePrimaryTextBlock	Seeks investment results that approximate, as closely as practicable and before expenses, the performance of the Russell Small Cap Completeness® Index.
Expense, Heading	rr_ExpenseHeading	Fees and Expenses
Expense, Narrative	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Fund shares.
Operating Expenses, Caption	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	rr_PortfolioTurnoverTextBlock	The Fund will pay 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.
Other Expenses, New Fund, Based on Estimates	rr_OtherExpensesNewFundBasedOnEstimates	Other expenses are based on estimated amounts for the current fiscal year.
Expense Example, Heading	rr_ExpenseExampleHeading	EXAMPLE
Expense Example, Narrative	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.
Expense Example, By Year, Caption	rr_ExpenseExampleByYearCaption	Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Investment Strategy, Heading	rr_StrategyHeading	Principal Investment Strategies
Investment Strategy, Narrative	rr_StrategyNarrativeTextBlock	The Fund is managed using a passive/indexing investment approach and invests substantially all of its assets in securities (mostly common stocks) of companies that are included (at the time of purchase) in the Russell Small Cap Completeness® Index (the Index). As of February 22, 2012 the market capitalization of the companies included in the Index ranged from $16 million to $41.5 billion. The Index is composed of securities of the companies included in the Russell 3000® Index (which includes the largest 3,000 U.S. companies), excluding the securities of companies that are constituents of the S&P 500 Index (which includes 500 leading U.S. companies). The Index is constructed to attempt to provide a comprehensive and unbiased barometer of the extended broad market of U.S. equity securities beyond that of the 500 leading U.S. companies included in the S&P 500 Index. The market capitalization range and the composition of the Index are subject to change. The Fund generally will attempt to invest in securities (including interests of real estate investment trusts (REITs)) composing the Index in approximately the same proportions as they are represented in the Index. The Fund's ability to fully replicate the performance of the Index will depend to some extent on the size and timing of cash flows into and out of the Fund, as well as on the level of the Fund's expenses. In some cases, it may not be possible or practicable to purchase all of the securities composing the Index or to hold them in the same weightings as they are represented in the Index. In those cases, the Fund's sub-adviser (the Sub-Adviser) may employ a sampling or optimization technique to construct the Fund's portfolio. In seeking to replicate the performance of the Index, the Fund may also invest in exchange-traded funds (ETFs) and REITs that are not constituents of the Index. The Sub-Adviser selects the Fund's securities under the general supervision of SEI Investments Management Corporation, the Fund's adviser (SIMC), but the Sub-Adviser makes no attempt to "manage" the Fund in the traditional sense (i.e., by using economic, market or financial analyses). Instead, the Sub-Adviser purchases a basket of securities that includes most of the companies in the Index. However, the Sub-Adviser may sell an investment if the merit of the investment has been substantially impaired by extraordinary events or adverse financial conditions. The Fund may, at times, purchase or sell index futures contracts, or options on those futures, or engage in forward or swap transactions in lieu of investing directly in the securities making up the Index or to enhance the Fund's replication of the Index's return. In addition, for liquidity purposes, the Fund may invest in securities that are not included in the Index, cash and cash equivalents or money market instruments, such as reverse repurchase agreements and money market funds. The Fund's return may not match the return of the Index. Russell Investment Group is the source and owner of the trademarks, service marks and copyrights related to Russell Indexes. Russell Small Cap Completeness® Index and Russell 3000® Index are trademarks of Russell Investment Group.
Risk, Heading	rr_RiskHeading	Principal Risks
Risk, Narrative	rr_RiskNarrativeTextBlock	Derivatives Risk - The Fund's use of futures, forwards, options and swaps is subject to market risk, leverage risk, correlation risk and liquidity risk. Market risk, leverage risk and liquidity risk are described below. Correlation risk is the risk that changes in the value of the derivative may not correlate perfectly with the underlying asset, rate or index. The Fund's use of forwards and swap agreements is also subject to credit risk and valuation risk. Valuation risk is the risk that the derivative may be difficult to value and/or valued incorrectly. Credit risk is the risk that the issuer of a security or the counterparty to a contract will default or otherwise become unable to honor a financial obligation. Each of these risks could cause the Fund to lose more than the principal amount invested in a derivative instrument. Equity Market Risk - The risk that stock prices will fall over short or extended periods of time. Exchange-Traded Funds (ETFs) Risk - The risks of owning shares of an ETF generally reflect the risks of owning the underlying securities the ETF is designed to track, although lack of liquidity in an ETF could result in its value being more volatile than the underlying portfolio securities.    Investment Style Risk - The risk that the Fund's investment approach, which attempts to replicate the performance of the Index, may underperform other segments of the equity markets or the equity markets as a whole. The Fund is also subject to the risk that the securities in which it invests may underperform other segments of the equity markets or the equity markets as a whole. Leverage Risk - The use of leverage can amplify the effects of market volatility on the Fund's share price and may also cause the Fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations. Liquidity Risk - The risk that certain securities may be difficult or impossible to sell at the time and the price that the seller would like. The seller may have to lower the price, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance. Market Risk - The risk that the market value of a security may move up and down, sometimes rapidly and unpredictably. Market risk may affect a single issuer, an industry, a sector or the equity market as a whole. Opportunity Risk - The risk of missing out on an investment opportunity because the assets necessary to take advantage of it are tied up in other investments. REITs Risk - REITs are trusts that invest primarily in commercial real estate or real estate-related loans. The Fund's investments in REITs will be subject to the risks associated with the direct ownership of real estate. Risks commonly associated with the direct ownership of real estate include fluctuations in the value of underlying properties, defaults by borrowers or tenants, changes in interest rates and risks related to general or local economic conditions. Some REITs may have limited diversification and may be subject to risks inherent in financing a limited number of properties. Sampling Risk - The Fund may not fully replicate the Index and may hold securities not included in the Index. As a result, the Fund may not track the return of the Index as well as it would have if the Fund purchased all of the securities in the Index. Small and Medium Capitalization Risk - The small and medium capitalization companies in which the Fund invests may be more vulnerable to adverse business or economic events than larger, more established companies. In particular, small and medium capitalization companies may have limited product lines, markets and financial resources and may depend upon a relatively small management group. Therefore, small and medium capitalization stocks may be more volatile than those of larger companies. Small and medium capitalization stocks may be traded over-the-counter or listed on an exchange. Tracking Error Risk - The risk that the Fund's performance may vary substantially from the performance of the Index as a result of cash flows, Fund expenses, imperfect correlation between the Fund's and Index's investments and other factors. Loss of money is a risk of investing in the Fund.
Risk, Lose Money	rr_RiskLoseMoney	Loss of money is a risk of investing in the Fund.
Bar Chart and Performance Table, Heading	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance, Narrative	rr_PerformanceNarrativeTextBlock	As of February 23, 2012, the Fund had not yet commenced operations and did not have a performance history.
Performance, One Year or Less	rr_PerformanceOneYearOrLess	As of February 23, 2012, the Fund had not yet commenced operations and did not have a performance history.
SIIT Extended Market Index Fund (Prospectus Summary) | SIIT Extended Market Index Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Management Fees	rr_ManagementFeesOverAssets	0.12%
Distribution (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.15%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.27%
Expense Example, With Redemption, 1 Year	rr_ExpenseExampleYear01	28
Expense Example, With Redemption, 3 Years	rr_ExpenseExampleYear03	87

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