| Label | Element | Value |
|---|---|---|
| Western Asset SMASh Series TF Fund | Municipal Bonds [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 94.20% |
| Western Asset SMASh Series TF Fund | Municipal Bonds Deposited in Tender Option Bond Trusts [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 2.30% |
| Western Asset SMASh Series TF Fund | Short-Term Investments [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.50% |