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SIIT SMALL MID CAP EQUITY FUND  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading SIIT SMALL MID CAP EQUITY FUND
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Long-term capital appreciation.
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 Fund shares.
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 116% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 116.00%
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.
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs and returns 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

Under normal circumstances, the Small/Mid Cap Equity Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities (including common and preferred stocks) of small and medium-sized companies, exchange-traded funds (ETFs) based on small and medium-sized capitalization indexes and securities of real estate investment trusts (REITs). The Fund will invest primarily in common stocks of U.S. companies with market capitalizations in the range of companies in the Russell 2500 Index (between $30 million and $15 billion as of July 31, 2019) at the time of purchase. The market capitalization range and the composition of the Russell 2500 Index are subject to change. The Fund's investments in equity securities may include, to a lesser extent, securities of large capitalization companies.


The Fund uses a multi-manager approach, relying upon one or more sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers) with differing investment philosophies to manage portions of the Fund's portfolio under the general supervision of SEI Investments Management Corporation (SIMC or the Adviser). Each Sub-Adviser, in managing its portion of the Fund's assets, generally applies a growth-oriented, a value-oriented or a blended approach to selecting investments. Growth-oriented managers generally select stocks they believe have attractive growth and appreciation potential in light of such characteristics as revenue and earnings growth, expectations from sell-side analysts and relative valuation, while value-oriented managers generally select stocks they believe are attractively valued in light of fundamental characteristics such as earnings, capital structure and/or return on invested capital. Due to its investment strategy, the Fund may buy and sell securities frequently.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

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.


Small and Medium Capitalization Risk — The risk that small and medium capitalization companies in which the Fund may invest 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 capitalization and medium capitalization stocks may be more volatile than those of larger companies. Small capitalization and medium capitalization stocks may be traded over-the-counter or listed on an exchange.


Investment Style Risk — The risk that smaller and medium capitalization securities may underperform other segments of the equity market or the equity markets as a whole.


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. When the Fund invests in an ETF, in addition to directly bearing the expenses associated with its own operations, it will bear a pro rata portion of the ETF's expenses.


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.


Liquidity Risk — The risk that certain securities may be difficult or impossible to sell at the time and the price that the Fund would like. The Fund may have to lower the price of the security, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance.


Portfolio Turnover Risk — Due to its investment strategy, the Fund may buy and sell securities frequently. This may result in higher transaction costs and additional capital gains tax liabilities, which may affect the Fund's performance.


Investing in the Fund involves risk, and there is no guarantee that the Fund will achieve its investment goal. You could lose money on your investment in the Fund, just as you could with other investments. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money on your investment in the Fund, just as you could with other investments.
RIsk Not Insured [Text] rr_RiskNotInsured An investment in the Fund 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 Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception compare with those of a broad measure of market performance. The performance information shown is based on full calendar years. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. For current performance information, please call 1-800-DIAL-SEI.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception compare with those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-DIAL-SEI
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

Best Quarter: 22.03% (6/30/09)
Worst Quarter: -21.44% (09/30/11)
The Fund's total return from January 1, 2019 to June 30, 2019 was 18.13%.
Year to Date Return, Label rr_YearToDateReturnLabel total return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2019
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 18.13%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 22.03%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (21.44%)
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2018)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
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 Your actual after-tax returns will depend on your 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 401(k) plans or individual retirement accounts.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

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. Your actual after-tax returns will depend on your 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 401(k) plans or individual retirement accounts. In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
SIIT SMALL MID CAP EQUITY FUND | Russell 2500 Index Return (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Russell 2500 Index Return (reflects no deduction for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01 (10.00%)
5 Years rr_AverageAnnualReturnYear05 5.15%
10 Years rr_AverageAnnualReturnYear10 13.15%
Since Inception rr_AverageAnnualReturnSinceInception 8.47%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 15, 2003
SIIT SMALL MID CAP EQUITY FUND | SIIT SMALL MID CAP EQUITY FUND - CLASS A  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.65%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.07%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.72%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 74
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 230
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 401
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 894
Annual Return 2009 rr_AnnualReturn2009 38.78%
Annual Return 2010 rr_AnnualReturn2010 25.08%
Annual Return 2011 rr_AnnualReturn2011 (4.52%)
Annual Return 2012 rr_AnnualReturn2012 15.63%
Annual Return 2013 rr_AnnualReturn2013 39.91%
Annual Return 2014 rr_AnnualReturn2014 7.10%
Annual Return 2015 rr_AnnualReturn2015 (4.24%)
Annual Return 2016 rr_AnnualReturn2016 16.62%
Annual Return 2017 rr_AnnualReturn2017 13.65%
Annual Return 2018 rr_AnnualReturn2018 (11.02%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (11.02%)
5 Years rr_AverageAnnualReturnYear05 3.88%
10 Years rr_AverageAnnualReturnYear10 12.48%
Since Inception rr_AverageAnnualReturnSinceInception 7.58%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 15, 2003
SIIT SMALL MID CAP EQUITY FUND | SIIT SMALL MID CAP EQUITY FUND - CLASS A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 (15.71%)
5 Years rr_AverageAnnualReturnYear05 0.18%
10 Years rr_AverageAnnualReturnYear10 9.75%
Since Inception rr_AverageAnnualReturnSinceInception 5.44%
SIIT SMALL MID CAP EQUITY FUND | SIIT SMALL MID CAP EQUITY FUND - CLASS A | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
1 Year rr_AverageAnnualReturnYear01 (3.90%)
5 Years rr_AverageAnnualReturnYear05 2.44%
10 Years rr_AverageAnnualReturnYear10 9.85%
Since Inception rr_AverageAnnualReturnSinceInception 5.53%