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SIIT U.S. Managed Volatility Fund  
Risk/Return: rr_RiskReturnAbstract  
Supplement to Prospectus [Text Block] rr_SupplementToProspectusTextBlock

SEI INSTITUTIONAL INVESTMENTS TRUST

SIIT U.S. Managed Volatility Fund
SIIT Global Managed Volatility Fund
(each, a "Fund" and, collectively, the "Funds")

Supplement dated November 13, 2020
to the Prospectus dated September 30, 2020

This Supplement provides new and additional information beyond that contained in the Prospectus and should be read in conjunction with such Prospectus.

The Prospectus is hereby amended and supplemented to reflect the following changes to the Funds.
Risk/Return [Heading] rr_RiskReturnHeading SIIT U.S. Managed Volatility Fund
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

Addition of Secondary Benchmark for the SIIT U.S. Managed Volatility Fund

In the Fund Summary for the Fund, under the heading titled "Performance Information," under the sub-heading titled "Average Annual Total Returns (for the periods ended December 31, 2019)," the following text is hereby added:
Performance Additional Market Index [Text] rr_PerformanceAdditionalMarketIndex This table compares the Fund's average annual total returns to those of a broad-based index and the Fund's 75/25 Blended Benchmark, which consists of the MSCI USA Minimum Volatility Index and the Russell 3000 Value Index. The Fund's Blended Benchmark is designed to provide a useful comparison to the Fund's overall performance and more accurately reflect the Fund's investment strategy than the broad-based index.
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

This table compares the Fund's average annual total returns to those of a broad-based index and the Fund's 75/25 Blended Benchmark, which consists of the MSCI USA Minimum Volatility Index and the Russell 3000 Value Index. The Fund's Blended Benchmark is designed to provide a useful comparison to the Fund's overall performance and more accurately reflect the Fund's investment strategy than the broad-based index.

In addition, under the same sub-heading, the table is hereby deleted and replaced with the following:
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

There are no other changes to the Prospectus.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE
SIIT U.S. Managed Volatility Fund | Russell 3000 Index Return (reflects no deduction for fees,Russell 3000 Index Return (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Russell 3000 Index Return (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 31.02%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.24%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.42%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 14.69%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2008
SIIT U.S. Managed Volatility Fund | The Fund's 75/25 Blended Benchmark Return (reflects noThe Fund's 75/25 Blended Benchmark Return (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel The Fund's 75/25 Blended Benchmark Return (reflects no deduction for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 27.63%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.54%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.70%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 14.16%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2008
SIIT U.S. Managed Volatility Fund | SIIT U.S. Managed Volatility Fund - Class A  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 23.94%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 10.01%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 13.51%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 13.67%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2008
SIIT U.S. Managed Volatility Fund | SIIT U.S. Managed Volatility Fund - Class A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 22.06%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 7.27%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 10.53%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 10.87%
SIIT U.S. Managed Volatility Fund | SIIT U.S. Managed Volatility 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
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.37%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 7.27%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 10.17%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 10.45%