Under normal circumstances, the fund invests at least 80% of its assets in municipal securities and other investments with similar economic characteristics, the interest on which is exempt from regular federal income tax but which may be subject to the federal alternative minimum tax. The fund’s 80% investment policy may not be changed without shareholder approval.

Municipal securities include debt obligations issued by any of the 50 states and certain other municipal issuers and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities issued or backed by banks, insurance companies and other financial institutions.

Although the fund may invest in securities of any maturity, it normally invests in intermediate-term and long-term municipal securities that have remaining maturities from one to more than thirty years at the time of purchase. The fund typically focuses on investment grade bonds (that is, securities rated in the Baa/BBB categories or above or, if unrated or deemed to be unrated by the subadviser, determined to be of comparable credit quality by the subadviser) but may from time to time invest, to no specified limit, in below investment grade bonds (commonly known as “high yield” or “junk” bonds).

Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself.

Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options, futures contracts and inverse floating rate instruments issued in tender option bond transactions, and other synthetic instruments that are intended to provide economic exposure to the securities or the issuer or to be used as a hedging technique. The fund may use one or more types of these instruments without limit, subject to applicable regulatory requirements. These instruments are taken into account when determining compliance with the fund’s 80% investment policy. For additional information regarding derivatives, see “More on the fund’s investment strategies, investments and risks—Derivatives” in this Prospectus.

The fund may also engage in a variety of transactions using derivatives in order to change the investment characteristics of its portfolio (such as shortening or lengthening duration) and for other purposes. The fund may leverage its assets by investing proceeds received through tender option bond transactions, which is considered a form of borrowing. See “More on the fund’s investment strategies, investments and risks—Tender option bonds” in the Prospectus.

The fund’s subadviser selects securities to buy and sell for the fund primarily by seeking to identify undervalued sectors and individual securities and securities it believes will benefit from changes in market conditions. It seeks to enhance returns and reduce risks by taking advantage of shifts in the municipal yield curve, credit quality spreads and variations in market sectors.

The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund.

S000052179 [Member] Investment Strategy - Western Asset SMASh Series TF Fund	Feb. 28, 2025
Prospectus [Line Items]
Strategy [Heading]	Principal investment strategies
Strategy Narrative [Text Block]	Under normal circumstances, the fund invests at least 80% of its assets in municipal securities and other investments with similar economic characteristics, the interest on which is exempt from regular federal income tax but which may be subject to the federal alternative minimum tax. The fund’s 80% investment policy may not be changed without shareholder approval. Municipal securities include debt obligations issued by any of the 50 states and certain other municipal issuers and their political subdivisions, agencies and public authorities, certain other governmental issuers (such as Puerto Rico, the U.S. Virgin Islands and Guam) and other qualifying issuers. These securities include participation or other interests in municipal securities issued or backed by banks, insurance companies and other financial institutions. Although the fund may invest in securities of any maturity, it normally invests in intermediate-term and long-term municipal securities that have remaining maturities from one to more than thirty years at the time of purchase. The fund typically focuses on investment grade bonds (that is, securities rated in the Baa/BBB categories or above or, if unrated or deemed to be unrated by the subadviser, determined to be of comparable credit quality by the subadviser) but may from time to time invest, to no specified limit, in below investment grade bonds (commonly known as “high yield” or “junk” bonds). Some municipal securities, such as general obligation issues, are backed by the issuer’s taxing authority, while other municipal securities, such as revenue issues, are backed only by revenues from certain facilities or other sources and not by the issuer itself. Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options, futures contracts and inverse floating rate instruments issued in tender option bond transactions, and other synthetic instruments that are intended to provide economic exposure to the securities or the issuer or to be used as a hedging technique. The fund may use one or more types of these instruments without limit, subject to applicable regulatory requirements. These instruments are taken into account when determining compliance with the fund’s 80% investment policy. For additional information regarding derivatives, see “More on the fund’s investment strategies, investments and risks—Derivatives” in this Prospectus. The fund may also engage in a variety of transactions using derivatives in order to change the investment characteristics of its portfolio (such as shortening or lengthening duration) and for other purposes. The fund may leverage its assets by investing proceeds received through tender option bond transactions, which is considered a form of borrowing. See “More on the fund’s investment strategies, investments and risks—Tender option bonds” in the Prospectus. The fund’s subadviser selects securities to buy and sell for the fund primarily by seeking to identify undervalued sectors and individual securities and securities it believes will benefit from changes in market conditions. It seeks to enhance returns and reduce risks by taking advantage of shifts in the municipal yield curve, credit quality spreads and variations in market sectors. The fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund.