The fund has a flexible investment strategy and invests in a variety of securities and instruments and uses a variety of investment techniques in pursuing its objective. The fund presently intends to limit its investments to U.S. dollar denominated securities and currently anticipates that it will generally only purchase debt securities that are rated in the Baa or BBB categories or above at the time of purchase by one or more Nationally Recognized Statistical Rating Organizations (“NRSROs”) or unrated securities of comparable quality at the time of purchase (as determined by the subadviser). These securities are known as “investment grade securities.” The fund may invest a substantial portion of its assets in mortgage-backed and asset-backed securities.

Currently the fund does not contemplate investing 25% or more of its assets in a single country or a small number of countries, except in the United States. The fund may invest in issuers located in emerging market countries. The subadviser may invest a significant portion of the fund’s assets in various industry sectors, to the extent consistent with the fund’s fundamental investment restrictions.

In purchasing debt obligations for the fund, the subadviser may take full advantage of the entire range of maturities and durations, and may adjust the average maturity or duration of the fund’s investments from time to time.

Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options, interest rate swaps, credit default swaps and options on credit default swaps, futures contracts, 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. For additional information regarding derivatives, see “More on the funds’ investment strategies, investments and risks—Derivatives” in the 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 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.

S000069099 [Member] Investment Strategy - WESTERN ASSET SMASh SERIES CORE COMPLETION FUND	Feb. 28, 2025
Prospectus [Line Items]
Strategy [Heading]	Principal investment strategies
Strategy Narrative [Text Block]	The fund has a flexible investment strategy and invests in a variety of securities and instruments and uses a variety of investment techniques in pursuing its objective. The fund presently intends to limit its investments to U.S. dollar denominated securities and currently anticipates that it will generally only purchase debt securities that are rated in the Baa or BBB categories or above at the time of purchase by one or more Nationally Recognized Statistical Rating Organizations (“NRSROs”) or unrated securities of comparable quality at the time of purchase (as determined by the subadviser). These securities are known as “investment grade securities.” The fund may invest a substantial portion of its assets in mortgage-backed and asset-backed securities. Currently the fund does not contemplate investing 25% or more of its assets in a single country or a small number of countries, except in the United States. The fund may invest in issuers located in emerging market countries. The subadviser may invest a significant portion of the fund’s assets in various industry sectors, to the extent consistent with the fund’s fundamental investment restrictions. In purchasing debt obligations for the fund, the subadviser may take full advantage of the entire range of maturities and durations, and may adjust the average maturity or duration of the fund’s investments from time to time. Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options, interest rate swaps, credit default swaps and options on credit default swaps, futures contracts, 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. For additional information regarding derivatives, see “More on the funds’ investment strategies, investments and risks—Derivatives” in the 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 borrow money to increase portfolio holdings, to the extent consistent with the fund’s fundamental investment restrictions. 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.