Under normal circumstances, the fund will invest its assets in debt obligations, other fixed income securities and related investments. The fund may invest in all types of U.S. dollar denominated short-term debt instruments, including bank obligations, commercial paper and asset-backed securities, mortgage-backed securities (including U.S. government and privately issued mortgage-backed securities, collateralized mortgage obligations and mortgage-related derivative securities), U.S. government agency or instrumentality securities, corporate loans, corporate debt securities, structured instruments and repurchase agreements.

The fund focuses on investment grade bonds (that is, securities rated in the Baa/BBB categories or above or, if unrated, determined to be of comparable credit quality by the fund’s subadviser), but may invest up to 10% of its assets in below investment grade bonds (commonly known as “high yield” or “junk” bonds).

Although the fund may invest in securities of any maturity, the fund normally expects to maintain a dollar-weighted average effective duration, as estimated by the subadviser of less than or equal to one year. Effective duration seeks to measure the expected sensitivity of market price to changes in interest rates, taking into account the anticipated effects of structural complexities (for example, some bonds can be prepaid by the issuer).

The fund may invest up to 25% of its assets in U.S. dollar-denominated securities of foreign issuers, including mortgage-backed securities and asset-backed securities issued by foreign entities.

Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options and 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 fund’s investment strategies, investments and risks—Derivatives” in the fund’s 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.

Investment Strategy - Western Asset Ultra-Short Income Fund	May 31, 2026
Prospectus [Line Items]
Strategy [Heading]	Principal investment strategies
Strategy Narrative [Text Block]	Under normal circumstances, the fund will invest its assets in debt obligations, other fixed income securities and related investments. The fund may invest in all types of U.S. dollar denominated short-term debt instruments, including bank obligations, commercial paper and asset-backed securities, mortgage-backed securities (including U.S. government and privately issued mortgage-backed securities, collateralized mortgage obligations and mortgage-related derivative securities), U.S. government agency or instrumentality securities, corporate loans, corporate debt securities, structured instruments and repurchase agreements. The fund focuses on investment grade bonds (that is, securities rated in the Baa/BBB categories or above or, if unrated, determined to be of comparable credit quality by the fund’s subadviser), but may invest up to 10% of its assets in below investment grade bonds (commonly known as “high yield” or “junk” bonds). Although the fund may invest in securities of any maturity, the fund normally expects to maintain a dollar-weighted average effective duration, as estimated by the subadviser of less than or equal to one year. Effective duration seeks to measure the expected sensitivity of market price to changes in interest rates, taking into account the anticipated effects of structural complexities (for example, some bonds can be prepaid by the issuer). The fund may invest up to 25% of its assets in U.S. dollar-denominated securities of foreign issuers, including mortgage-backed securities and asset-backed securities issued by foreign entities. Instead of, and/or in addition to, investing directly in particular securities, the fund may use instruments such as derivatives, including options and 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 fund’s investment strategies, investments and risks—Derivatives” in the fund’s 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 buy additional securities or for other purposes.
Summary of Selection Criteria for Rule 35d-1 Term in Fund Name [Text Block]	The fund focuses on investment grade bonds (that is, securities rated in the Baa/BBB categories or above or, if unrated, determined to be of comparable credit quality by the fund’s subadviser), but may invest up to 10% of its assets in below investment grade bonds (commonly known as “high yield” or “junk” bonds).