Label	Element	Value
Class A Prospectus \| SIMT SMALL CAP GROWTH FUND
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	ck0000804239_SupplementTextBlock	SEI INSTITUTIONAL MANAGED TRUST Large Cap Value Fund Small Cap Fund Small Cap Value Fund Small Cap Growth Fund Tax-Managed Small/Mid Cap Fund (the "Funds") Supplement dated January 15, 2016 to the Class A Shares Prospectus (the "Prospectus") dated January 31, 2015, as amended on July 17, 2015, November 6, 2015 and December 17, 2015 This Supplement provides new and additional information beyond that contained in the Class A Shares 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	SIMT SMALL CAP GROWTH FUND
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Changes to the Small Cap Growth Fund's Principal Investment Strategies and Sub-Advisers In the Small Cap Growth Fund's "Principal Investment Strategies" section, the text is hereby deleted and replaced with the following: Under normal circumstances, the Small Cap Growth Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of small companies. The Fund will primarily seek to purchase securities believed to have attractive growth and appreciation potential. For the 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 $20 million and $7.3 billion as of December 31, 2014), as determined 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 common and preferred stocks, warrants and, to a lesser extent, real estate investment trusts (REITs), exchange-traded funds (ETFs) and securities of large capitalization companies. The Fund uses a multi-manager approach, relying primarily upon a number of sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers) to manage portions of the Fund's portfolio under the general supervision of SEI Investments Management Corporation, the Fund's adviser (SIMC or the Adviser). Assets of the Fund not allocated to a Sub-Adviser are managed directly by SIMC. In managing its portion of the Fund's assets, SIMC or each Sub-Adviser may apply a variety of quantitative and/or fundamental investment styles. A quantitative investment style generally involves a systematic or rules-based approach to selecting investments based on specific measurable factors. A fundamental investment style generally involves selecting investments through research and analysis of financial statements, relevant industry and economic data, or other characteristics. The Fund implements the investment recommendations of SIMC and its Sub-Advisers through the use of an overlay manager appointed by SIMC. Each Sub-Adviser provides a model portfolio to the Fund on an ongoing basis that represents that Sub-Adviser's recommendation as to the securities to be purchased, sold or retained by the Fund. The overlay manager then constructs a portfolio for the Fund that represents the aggregation of the model portfolios of the Sub-Advisers and SIMC with the weighting of each Sub-Adviser's model in the total portfolio determined by SIMC. Pursuant to direction from SIMC, the overlay manager has limited authority to vary from the models. For example, SIMC may direct the overlay manager to adjust the portfolio to implement SIMC's forward looking views regarding various portfolio characteristics or factors, or for risk management purposes. The overlay manager may also vary the portfolio implementation to seek trading cost efficiencies, loss harvesting, portfolio rebalancing or other portfolio construction objectives as directed by SIMC. There are no other changes to the principal investment strategies or portfolio management of the Small Cap Growth Fund. PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

Label

Element

Value

Class A Prospectus | SIMT SMALL CAP GROWTH FUND

rr_RiskReturnAbstract

ck0000804239_SupplementTextBlock

SEI INSTITUTIONAL MANAGED TRUST

Large Cap Value Fund
Small Cap Fund
Small Cap Value Fund
Small Cap Growth Fund
Tax-Managed Small/Mid Cap Fund
(the "Funds")

Supplement dated January 15, 2016
to the Class A Shares Prospectus (the "Prospectus") dated January 31, 2015, as amended on
July 17, 2015, November 6, 2015 and December 17, 2015

This Supplement provides new and additional information beyond that contained in the Class A Shares 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

SIMT SMALL CAP GROWTH FUND

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

Changes to the Small Cap Growth Fund's Principal Investment Strategies and Sub-Advisers

In the Small Cap Growth Fund's "Principal Investment Strategies" section, the text is hereby deleted and replaced with the following:

Under normal circumstances, the Small Cap Growth Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of small companies. The Fund will primarily seek to purchase securities believed to have attractive growth and appreciation potential. For the 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 $20 million and $7.3 billion as of December 31, 2014), as determined 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 common and preferred stocks, warrants and, to a lesser extent, real estate investment trusts (REITs), exchange-traded funds (ETFs) and securities of large capitalization companies.

The Fund uses a multi-manager approach, relying primarily upon a number of sub-advisers (each, a Sub-Adviser and collectively, the Sub-Advisers) to manage portions of the Fund's portfolio under the general supervision of SEI Investments Management Corporation, the Fund's adviser (SIMC or the Adviser). Assets of the Fund not allocated to a Sub-Adviser are managed directly by SIMC. In managing its portion of the Fund's assets, SIMC or each Sub-Adviser may apply a variety of quantitative and/or fundamental investment styles. A quantitative investment style generally involves a systematic or rules-based approach to selecting investments based on specific measurable factors. A fundamental investment style generally involves selecting investments through research and analysis of financial statements, relevant industry and economic data, or other characteristics.

The Fund implements the investment recommendations of SIMC and its Sub-Advisers through the use of an overlay manager appointed by SIMC. Each Sub-Adviser provides a model portfolio to the Fund on an ongoing basis that represents that Sub-Adviser's recommendation as to the securities to be purchased, sold or retained by the Fund. The overlay manager then constructs a portfolio for the Fund that represents the aggregation of the model portfolios of the Sub-Advisers and SIMC with the weighting of each Sub-Adviser's model in the total portfolio determined by SIMC.

Pursuant to direction from SIMC, the overlay manager has limited authority to vary from the models. For example, SIMC may direct the overlay manager to adjust the portfolio to implement SIMC's forward looking views regarding various portfolio characteristics or factors, or for risk management purposes. The overlay manager may also vary the portfolio implementation to seek trading cost efficiencies, loss harvesting, portfolio rebalancing or other portfolio construction objectives as directed by SIMC.

There are no other changes to the principal investment strategies or portfolio management of the Small Cap Growth Fund.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE