v3.26.3
Label Element Value
SIIT Small Cap II Fund | SMALL CAP II FUND  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk/Return [Heading] oef_RiskReturnHeading SMALL CAP II FUND
Objective [Heading] oef_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] oef_ObjectivePrimaryTextBlock

Capital appreciation.
Expense Heading [Optional Text] oef_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] oef_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy, hold, and sell Fund shares. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table and examples below.
Operating Expenses Caption [Optional Text] oef_OperatingExpensesCaption ANNUAL FUND OPERATING EXPENSES (expenses that you pay each year as a percentage of the value of your investment)
Expense Example [Heading] oef_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] oef_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 or hold 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] oef_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Portfolio Turnover [Heading] oef_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] oef_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. During the most recent fiscal year, the Fund’s portfolio turnover rate was 104% of the average value of its portfolio.
Portfolio Turnover, Rate oef_PortfolioTurnoverRate 104.00%
Strategy [Heading] oef_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] oef_StrategyNarrativeTextBlock

Under normal circumstances, the Small Cap II 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 companies, exchange-traded funds (ETFs) based on small capitalization indexes and securities of real estate investment trusts (REITs). For purposes of this Fund, a small company is a company with a market capitalization in the range of companies in the Russell 2000 Index (between $6.99 million and $13.78 billion as of July 31, 2026) at the time of purchase. The market capitalization range and the composition of the

Russell 2000 Index are subject to change. The Fund’s investments in equity securities may include, to a lesser extent, securities of medium and 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.
Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] fnd_NmRule35d1TermDfnSmryTextBlock For purposes of this Fund, a small company is a company with a market capitalization in the range of companies in the Russell 2000 Index (between $6.99 million and $13.78 billion as of July 31, 2026) at the time of purchase. The market capitalization range and the composition of the Russell 2000 Index are subject to change. The Fund’s investments in equity securities may include, to a lesser extent, securities of medium and large capitalization companies.
Summary of Selection Criteria for Rule 35d-1 Term in Fund Name [Text Block] fnd_NmRule35d1TermSlctnCritSmryTextBlock 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.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] fnd_NmRule35d1EightyPctInvstmntPlcyTextBlock Under normal circumstances, the Small Cap II 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 companies, exchange-traded funds (ETFs) based on small capitalization indexes and securities of real estate investment trusts (REITs).
Bar Chart and Performance Table [Heading] oef_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] oef_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] oef_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 Additional Market Index [Text] oef_PerformanceAdditionalMarketIndex This table compares the Fund’s average annual total returns to those of a broad-based securities market index and an additional index with characteristics relevant to the Fund’s investment strategy.
Performance Availability Phone [Text] oef_PerformanceAvailabilityPhone 1‑800-DIAL-SEI
Performance Past Does Not Indicate Future [Text] oef_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] oef_BarChartClosingTextBlock
Best Quarter: 30.14% (12/31/20)
Worst Quarter: -31.16% (3/31/20)
The Fund's total return from January 1, 2026 to June 30, 2026 was 20.23%.
Year to Date Return, Label [Optional Text] oef_YearToDateReturnLabel total return
Bar Chart, Year to Date Return, Date oef_BarChartYearToDateReturnDate Jun. 30, 2026
Bar Chart, Year to Date Return oef_BarChartYearToDateReturn 20.23%
Highest Quarterly Return, Label [Optional Text] oef_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date oef_BarChartHighestQuarterlyReturnDate Dec. 31, 2020
Highest Quarterly Return oef_BarChartHighestQuarterlyReturn 30.14%
Lowest Quarterly Return, Label [Optional Text] oef_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date oef_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return oef_BarChartLowestQuarterlyReturn (31.16%)
Performance Table Heading oef_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2025)
Index No Deduction for Fees, Expenses, or Taxes [Text] oef_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
Performance Table Uses Highest Federal Rate oef_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 oef_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 Narrative oef_PerformanceTableNarrativeTextBlock

This table compares the Fund’s average annual total returns to those of a broad-based securities market index and an additional index with characteristics relevant to the Fund’s investment strategy.

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.
SIIT Small Cap II Fund | SMALL CAP II FUND | Risk Lose Money [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock You could lose money on your investment in the Fund, just as you could with other investments.
SIIT Small Cap II Fund | SMALL CAP II FUND | Risk Not Insured [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock 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.
SIIT Small Cap II Fund | SMALL CAP II FUND | Market Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

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. Equity markets may decline significantly in response to adverse issuer, political, regulatory, market, economic or other developments that may cause broad changes in market value, public perceptions concerning these developments, and adverse investor sentiment or publicity. Similarly, environmental and public health risks, such as natural disasters, epidemics, pandemics or widespread fear that such events may occur, may impact markets adversely and cause market volatility in both the short- and long-term.
SIIT Small Cap II Fund | SMALL CAP II FUND | Small and Medium Capitalization Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Small and Medium Capitalization Risk — The 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 and medium capitalization stocks may be more volatile than those of larger companies. Small and medium capitalization stocks may be traded over-the-counter (OTC). OTC stocks may trade less frequently and in smaller volume than exchange listed stocks and may have more price volatility than that of exchange-listed stocks.
SIIT Small Cap II Fund | SMALL CAP II FUND | Investment Style Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Investment Style Risk — The risk that the securities in which the Fund principally invests may underperform other segments of the equity markets or the equity markets as a whole.
SIIT Small Cap II Fund | SMALL CAP II FUND | Preferred Stock Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Preferred Stock Risk — Preferred stock represents an equity or ownership interest in an issuer that pays dividends at a specified rate and that has precedence over common stock in the payment of dividends. In the event an issuer is liquidated or declares bankruptcy, the claims of owners of bonds take precedence over the claims of those who own preferred and common stock.
SIIT Small Cap II Fund | SMALL CAP II FUND | Exchange-Traded Funds Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Exchange-Traded Funds 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.
SIIT Small Cap II Fund | SMALL CAP II FUND | REITs Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

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.
SIIT Small Cap II Fund | SMALL CAP II FUND | Large Capitalization Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Large Capitalization Risk — The risk that larger, more established companies may be unable to respond quickly to new competitive challenges such as changes in technology and consumer tastes. Larger companies also may not be able to attain the high growth rates of successful smaller companies.
SIIT Small Cap II Fund | SMALL CAP II FUND | Quantitative Investing Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Quantitative Investing Risk — A quantitative investment style generally involves the use of computers to implement a systematic or rules-based approach to selecting investments based on specific measurable factors. Due to the significant role technology plays in such strategies, they carry the risk of unintended or unrecognized issues or flaws in the design, coding, implementation or maintenance of the computer programs or technology used in the development and implementation of the quantitative strategy.
SIIT Small Cap II Fund | SMALL CAP II FUND | Liquidity Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

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.
SIIT Small Cap II Fund | SMALL CAP II FUND | Opportunity Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

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.
SIIT Small Cap II Fund | SMALL CAP II FUND | Portfolio Turnover Risk [Member]  
Prospectus [Line Items] oef_ProspectusLineItems  
Risk [Text Block] oef_RiskTextBlock

Portfolio Turnover Risk — Due to its investment strategy, the Fund may buy and sell securities frequently. This may result in higher transaction costs and taxes subject to ordinary income tax rates as opposed to more favorable capital gains rates, which may affect the Fund’s performance.
SIIT Small Cap II Fund | SMALL CAP II FUND | SIIT Small Cap II Fund - Class A  
Prospectus [Line Items] oef_ProspectusLineItems  
Management Fees (as a percentage of Assets) oef_ManagementFeesOverAssets 0.65%
Distribution and Service (12b-1) Fees oef_DistributionAndService12b1FeesOverAssets 0.00%
Other Expenses (as a percentage of Assets): oef_OtherExpensesOverAssets 0.07%
Expenses (as a percentage of Assets) oef_ExpensesOverAssets 0.72%
Expense Example, with Redemption, 1 Year oef_ExpenseExampleYear01 $ 74
Expense Example, with Redemption, 3 Years oef_ExpenseExampleYear03 230
Expense Example, with Redemption, 5 Years oef_ExpenseExampleYear05 401
Expense Example, with Redemption, 10 Years oef_ExpenseExampleYear10 $ 894
SMALL CAP II FUND | Russell 2000 Index Return (reflects no deduction for fees, expenses or taxes)  
Prospectus [Line Items] oef_ProspectusLineItems  
Average Annual Return, Label [Optional Text] oef_AverageAnnualReturnLabel Russell 2000 Index Return (reflects no deduction for fees, expenses or taxes)
Average Annual Return, Percent oef_AvgAnnlRtrPct 12.81%
Average Annual Return, Percent oef_AvgAnnlRtrPct 6.09%
Average Annual Return, Percent oef_AvgAnnlRtrPct 9.62%
Average Annual Return, Percent oef_AvgAnnlRtrPct 10.25%
SMALL CAP II FUND | S&P 500 Index Return (reflects no deduction for fees, expenses or taxes)  
Prospectus [Line Items] oef_ProspectusLineItems  
Average Annual Return, Label [Optional Text] oef_AverageAnnualReturnLabel S&P 500 Index Return (reflects no deduction for fees, expenses or taxes)
Average Annual Return, Percent oef_AvgAnnlRtrPct 17.88%
Average Annual Return, Percent oef_AvgAnnlRtrPct 14.42%
Average Annual Return, Percent oef_AvgAnnlRtrPct 14.82%
Average Annual Return, Percent oef_AvgAnnlRtrPct 14.58%
SMALL CAP II FUND | SIIT Small Cap II Fund - Class A  
Prospectus [Line Items] oef_ProspectusLineItems  
Annual Return [Percent] oef_AnnlRtrPct 15.44%
Annual Return [Percent] oef_AnnlRtrPct 12.09%
Annual Return [Percent] oef_AnnlRtrPct (9.47%)
Annual Return [Percent] oef_AnnlRtrPct 25.02%
Annual Return [Percent] oef_AnnlRtrPct 19.84%
Annual Return [Percent] oef_AnnlRtrPct 20.93%
Annual Return [Percent] oef_AnnlRtrPct (15.39%)
Annual Return [Percent] oef_AnnlRtrPct 14.96%
Annual Return [Percent] oef_AnnlRtrPct 13.45%
Annual Return [Percent] oef_AnnlRtrPct 12.90%
Average Annual Return, Label [Optional Text] oef_AverageAnnualReturnLabel Return Before Taxes
Average Annual Return, Percent oef_AvgAnnlRtrPct 12.90%
Average Annual Return, Percent oef_AvgAnnlRtrPct 8.54%
Average Annual Return, Percent oef_AvgAnnlRtrPct 10.21%
Average Annual Return, Percent oef_AvgAnnlRtrPct 10.62%
Performance Inception Date oef_PerfInceptionDate Apr. 10, 2012
SMALL CAP II FUND | SIIT Small Cap II Fund - Class A | After Taxes on Distributions  
Prospectus [Line Items] oef_ProspectusLineItems  
Average Annual Return, Label [Optional Text] oef_AverageAnnualReturnLabel Return After Taxes on Distributions
Average Annual Return, Percent oef_AvgAnnlRtrPct 10.32%
Average Annual Return, Percent oef_AvgAnnlRtrPct 5.51%
Average Annual Return, Percent oef_AvgAnnlRtrPct 7.48%
Average Annual Return, Percent oef_AvgAnnlRtrPct 7.90%
SMALL CAP II FUND | SIIT Small Cap II Fund - Class A | After Taxes on Distributions and Sales  
Prospectus [Line Items] oef_ProspectusLineItems  
Average Annual Return, Label [Optional Text] oef_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
Average Annual Return, Percent oef_AvgAnnlRtrPct 9.21%
Average Annual Return, Percent oef_AvgAnnlRtrPct 5.89%
Average Annual Return, Percent oef_AvgAnnlRtrPct 7.39%
Average Annual Return, Percent oef_AvgAnnlRtrPct 7.78%