| Label | Element | Value |
|---|---|---|
| Western Asset SMASh Series M Fund | Collateralized Mortgage Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 34.90% |
| Western Asset SMASh Series M Fund | Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 29.90% |
| Western Asset SMASh Series M Fund | Asset-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 23.20% |
| Western Asset SMASh Series M Fund | U.S. Government & Agency Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 9.00% |
| Western Asset SMASh Series M Fund | Short-Term Investments [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.00% |