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SIIT Opportunistic Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading SIIT Opportunistic Income Fund
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Capital appreciation and income.
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 26% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 26.00%
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

The Opportunistic Income Fund invests primarily in a diversified portfolio of investment grade and non-investment grade fixed-income securities (junk bonds), including: (i) securities issued or guaranteed by the U.S. Government and its agencies and instrumentalities and obligations of U.S. and foreign commercial banks, such as certificates of deposit, time deposits, bankers' acceptances and bank notes; (ii) obligations of foreign governments; (iii) U.S. and foreign corporate debt securities, including commercial paper, and fully-collateralized repurchase agreements with counterparties deemed credit-worthy by the Fund's sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers); and (iv) securitized issues, such as residential and commercial mortgage-backed securities, asset-backed securities, mortgage dollar rolls, when issued/delayed delivery securities and collateralized debt obligations. These securities may be fixed-, variable- or floating-rate obligations and will primarily be rated CCC- or higher at the time of purchase by at least one ratings agency, although the Fund may also invest in lower rated securities. There are no restrictions on the maturity of any individual securities or on the Fund's average portfolio maturity, although the average portfolio duration of the Fund will typically vary between zero and two years. Duration is a measure of the expected life of a fixed income security that is used to determine the sensitivity of a security's price to changes in interest rates.


The Fund uses a multi-manager approach under the general supervision of SEI Investments Management Corporation (SIMC or the Adviser), allocating the assets among multiple Sub-Advisers that use different investment strategies designed to produce a total return that exceeds the total return of the 3-Month London Interbank Offered Rate (LIBOR). LIBOR is based on rates that contributor banks in London charge each other for interbank deposits and is typically used to set coupon rates on floating rate debt securities.


The Sub-Advisers may seek to enhance the Fund's return by actively managing the Fund's foreign currency exposure. Up to 10% of the Fund's assets may be invested in foreign currencies. In managing the Fund's currency exposure, the Sub-Advisers buy and sell currencies (i.e., take long or short positions) using derivatives, principally futures, foreign currency forward contracts, swaps and options. The Fund may take long and short positions in foreign currencies in excess of the value of the Fund's assets denominated in a particular currency or when the Fund does not own assets denominated in that currency. In managing the Fund's currency exposure for foreign securities, the Sub-Advisers may buy and sell currencies for hedging or for speculative purposes.


The Fund also invests a portion of its assets in bank loans, which are generally non-investment grade (junk bond) floating rate instruments. Up to 100% of the bank loans in which the Fund invests may be junk bonds. The Fund may invest in bank loans in the form of participations in the loans (participations) and assignments of all or a portion of the loans from third parties (assignments). The Fund may also invest in other financial instruments or use other investment techniques, such as reverse repurchase agreements, to seek to obtain market exposure to the securities in which the Fund primarily invests.


The Fund may also invest in futures contracts and swaps for speculative or hedging purposes. Futures and swaps are used to synthetically obtain exposure to securities or baskets of securities and to manage the Fund's interest rate duration and yield curve exposure. These derivatives are also used to mitigate the Fund's overall level of risk and/or the Fund's risk to particular types of securities or market segments. Interest rate swaps are further used to manage the Fund's yield spread sensitivity. When the Fund seeks to take an active long or short position with respect to the likelihood of an event of default of a security or basket of securities, the Fund may use credit default swaps. The Fund may buy credit default swaps in an attempt to manage credit risk where the Fund has credit exposure to an issuer, and the Fund may sell credit default swaps to more efficiently gain credit exposure to such security or basket of securities.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

Market Risk — The prices of the Fund's fixed income securities respond to economic developments, particularly interest rate changes, as well as to perceptions about the creditworthiness of individual issuers, including governments and their agencies. Generally, the Fund's fixed income securities will decrease in value if interest rates rise and vice versa. In a low interest rate environment, risks associated with rising rates are heightened. Declines in dealer market-making capacity as a result of structural or regulatory changes could decrease liquidity and/or increase volatility in the fixed income markets. In the case of foreign securities, price fluctuations will reflect international economic and political events, as well as changes in currency valuations relative to the U.S. dollar. In response to these events, the Fund's value may fluctuate and/or the Fund may experience increased redemptions from shareholders, which may impact the Fund's liquidity or force the Fund to sell securities into a declining or illiquid market.


Investment Style Risk — The risk that short-duration fixed income securities may underperform other segments of the fixed income markets or the fixed income markets as a whole.


Interest Rate Risk — The risk that a rise in interest rates will cause a fall in the value of fixed income securities, including U.S. Government securities, in which a Fund invests. A low interest rate environment may present greater interest rate risk, because there may be a greater likelihood of rates increasing and rates may increase more rapidly.


U.S. Government Securities Risk — Although U.S. Government securities are considered to be among the safest investments, they are not guaranteed against price movements due to changing interest rates. Obligations issued by some U.S. Government agencies are backed by the U.S. Treasury, while others are backed solely by the ability of the agency to borrow from the U.S. Treasury or by the agency's own resources.


Below Investment Grade Securities Risk — Fixed income securities rated below investment grade (junk bonds) involve greater risks of default or downgrade and are generally more volatile than investment grade securities because the prospect for repayment of principal and interest of many of these securities is speculative.


Foreign Investment/Emerging Markets Risk — The risk that non-U.S. securities may be subject to additional risks due to, among other things, political, social and economic developments abroad, currency movements, and different legal, regulatory and tax environments. These additional risks may be heightened with respect to emerging market countries because political turmoil and rapid changes in economic conditions are more likely to occur in these countries.


Bank Loans Risk — With respect to bank loans, the Fund will assume the credit risk of both the borrower of the loan and the lender that is selling the participation in the loan. The Fund may also have difficulty disposing of bank loans because, in certain cases, the market for such instruments is not highly liquid.


Corporate Fixed Income Securities Risk — Corporate fixed income securities respond to economic developments, especially changes in interest rates, as well as perceptions of the creditworthiness and business prospects of individual issuers.


Commercial Paper Risk — Commercial paper is a short-term obligation with a maturity generally ranging from one to 270 days and is issued by U.S. or foreign companies or other entities in order to finance their current operations. Such investments are unsecured and usually discounted from their value at maturity. The value of commercial paper may be affected by changes in the credit rating or financial condition of the issuing entities and will tend to fall when interest rates rise and rise when interest rates fall. Asset-backed commercial paper may be issued by structured investment vehicles or other conduits that are organized to issue the commercial paper and to purchase trade receivables or other financial assets. The repayment of asset-backed commercial paper depends primarily on the cash collections received from such an issuer's underlying asset portfolio and the issuer's ability to issue new asset-backed commercial paper.


Collateralized Debt Obligations (CDOs) and Collateralized Loan Obligations(CLOs) Risk — CDOs and CLOs are securities backed by an underlying portfolio of debt and loan obligations, respectively. CDOs and CLOs issue classes or "tranches" that vary in risk and yield and may experience substantial losses due to actual defaults, decrease in market value due to collateral defaults and removal of subordinate tranches, market anticipation of defaults and investor aversion to CDO and CLO securities as a class. The risks of investing in CDOs and CLOs depend largely on the tranche invested in and the type of the underlying debts and loans in the tranche of the CDO or CLO, respectively, in which the Fund invests. CDOs and CLOs also carry risks including, but not limited to, interest rate risk and credit risk, which are described below. For example, a liquidity crisis in the global credit markets could cause substantial fluctuations in prices for leveraged loans and high-yield debt securities and limited liquidity for such instruments. When the Fund invests in CDOs or CLOs, in addition to directly bearing the expenses associated with its own operations, it may bear a pro rata portion of the CDO's or CLO's expenses.


Repurchase Agreement Risk — Although repurchase agreement transactions must be fully collateralized at all times, they generally create leverage and involve some counterparty risk to the Fund whereby a defaulting counterparty could delay or prevent the Fund's recovery of collateral.


Asset-Backed Securities Risk — Payment of principal and interest on asset-backed securities is dependent largely on the cash flows generated by the assets backing the securities. Securitization trusts generally do not have any assets or sources of funds other than receivables and related property they own, and asset-backed securities are generally not insured or guaranteed by the related sponsor or any other entity. Asset-backed securities may be more illiquid than more conventional types of fixed income securities that the Fund may acquire.


Mortgage-Backed Securities Risk — Mortgage-backed securities are affected significantly by the rate of prepayments and modifications of the mortgage loans backing those securities, as well as by other factors such as borrower defaults, delinquencies, realized or liquidation losses and other shortfalls. Mortgage-backed securities are particularly sensitive to prepayment risk, which is described below, given that the term to maturity for mortgage loans is generally substantially longer than the expected lives of those securities; however, the timing and amount of prepayments cannot be accurately predicted. The timing of changes in the rate of prepayments of the mortgage loans may significantly affect the Fund's actual yield to maturity on any mortgage-backed securities, even if the average rate of principal payments is consistent with the Fund's expectation. Along with prepayment risk, mortgage-backed securities are significantly affected by interest rate risk, which is described above. In a low interest rate environment, mortgage loan prepayments would generally be expected to increase due to factors such as refinancings and loan modifications at lower interest rates. In contrast, if prevailing interest rates rise, prepayments of mortgage loans would generally be expected to decline and therefore extend the weighted average lives of mortgage-backed securities held or acquired by the Fund.


Mortgage Dollar Rolls Risk — Mortgage dollar rolls are transactions in which the Fund sells securities (usually mortgage-backed securities) and simultaneously contracts to repurchase substantially similar, but not identical, securities on a specified future date. If the broker-dealer to whom the Fund sells the security becomes insolvent, the Fund's right to repurchase the security may be restricted. Other risks involved in entering into mortgage dollar rolls include the risk that the value of the security may change adversely over the term of the mortgage dollar roll and that the security the Fund is required to repurchase may be worth less than the security that the Fund originally held.


Derivatives Risk — The Fund's use of futures contracts, forwards contracts, options and swaps is subject to market risk, leverage risk, correlation risk and liquidity risk. Credit risk, leverage risk and liquidity risk are described below. Many over-the-counter (OTC) derivative instruments will not have liquidity beyond the counterparty to the instrument. Market risk is the risk that the market value of an investment may move up and down, sometimes rapidly and unpredictably. 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 forward contracts, options and swaps 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. Market risk is described above. Each of these risks could cause the Fund to lose more than the principal amount invested in a derivative instrument. Some derivatives have the potential for unlimited loss, regardless of the size of the Fund's initial investment. The other parties to certain derivative contracts present the same types of credit risk as issuers of fixed income securities. The Fund's use of derivatives may also increase the amount of taxes payable by shareholders. Both U.S. and non-U.S. regulators are in the process of adopting and implementing regulations governing derivatives markets, the ultimate impact of which remains unclear.


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.


Currency Risk — Currency risk is the risk that foreign currencies will decline in value relative to the U.S. dollar or, in the case of hedging positions, that the U.S. dollar will decline in value relative to the currency hedged. Due to the Fund's active positions in currencies, it will be subject to the risk that currency exchange rates may fluctuate in response to, among other things, changes in interest rates, intervention (or failure to intervene) by U.S. or foreign governments, central banks or supranational entities, or by the imposition of currency controls or other political developments in the United States or abroad.


Credit Risk — 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.


Duration Risk — The longer-term securities in which the Fund may invest are more volatile. A portfolio with a longer average portfolio duration is more sensitive to changes in interest rates than a portfolio with a shorter average portfolio duration.


Extension Risk — The risk that rising interest rates may extend the duration of a fixed income security, typically reducing the security's value.


Prepayment Risk — The risk that, in a declining interest rate environment, fixed income securities with stated interest rates may have the principal paid earlier than expected, requiring the Fund to invest the proceeds at generally lower interest rates.


Leverage Risk — The Fund's use of derivatives and repurchase agreements (which effectively constitute a form of borrowing) may result in the Fund's total investment exposure substantially exceeding the value of its portfolio securities and the Fund's investment returns depending substantially on the performance of securities that the Fund may not directly own. 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. The Fund's use of leverage may result in a heightened risk of investment loss.


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: 5.76% (9/30/09)
Worst Quarter: -1.57% (9/30/11)
The Fund's total return from January 1, 2019 to June 30, 2019 was 3.61%.
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 3.61%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 5.76%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (1.57%)
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 deductions 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 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.
SIIT Opportunistic Income Fund | ICE BofAML USD 3-Month Deposit Offered Rate Constant Maturity Index (reflects no deductions for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel ICE BofAML USD 3-Month Deposit Offered Rate Constant Maturity Index (reflects no deductions for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01 2.08%
5 Years rr_AverageAnnualReturnYear05 0.86%
10 Years rr_AverageAnnualReturnYear10 0.67%
Since Inception rr_AverageAnnualReturnSinceInception 1.34%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 14, 2006
SIIT Opportunistic Income Fund | SIIT Opportunistic Income Fund - Class A  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.45%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.07%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.52%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 53
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 167
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 291
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 653
Annual Return 2009 rr_AnnualReturn2009 15.47%
Annual Return 2010 rr_AnnualReturn2010 6.96%
Annual Return 2011 rr_AnnualReturn2011 1.19%
Annual Return 2012 rr_AnnualReturn2012 7.14%
Annual Return 2013 rr_AnnualReturn2013 2.58%
Annual Return 2014 rr_AnnualReturn2014 2.07%
Annual Return 2015 rr_AnnualReturn2015 0.91%
Annual Return 2016 rr_AnnualReturn2016 4.20%
Annual Return 2017 rr_AnnualReturn2017 3.92%
Annual Return 2018 rr_AnnualReturn2018 1.82%
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 1.82%
5 Years rr_AverageAnnualReturnYear05 2.58%
10 Years rr_AverageAnnualReturnYear10 4.55%
Since Inception rr_AverageAnnualReturnSinceInception 0.93%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 14, 2006
SIIT Opportunistic Income Fund | SIIT Opportunistic Income Fund - Class A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 0.22%
5 Years rr_AverageAnnualReturnYear05 1.30%
10 Years rr_AverageAnnualReturnYear10 3.52%
Since Inception rr_AverageAnnualReturnSinceInception (0.11%)
SIIT Opportunistic Income Fund | SIIT Opportunistic Income 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 1.08%
5 Years rr_AverageAnnualReturnYear05 1.40%
10 Years rr_AverageAnnualReturnYear10 3.13%
Since Inception rr_AverageAnnualReturnSinceInception 0.27%