| Form N-CSR Cover | 6 Months Ended |
| Aug. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | Legg Mason Partners Institutional Trust |
| Entity Central Index Key | 0000889512 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Aug. 31, 2024 |
| Shareholder Report | 6 Months Ended | |
| Aug. 31, 2024 USD ($) $ / shares | ||
| Shareholder Report [Line Items] | ||
| Document Type | N-CSRS | |
| Amendment Flag | false | |
| Registrant Name | Legg Mason Partners Institutional Trust | |
| Entity Central Index Key | 0000889512 | |
| Entity Investment Company Type | N-1A | |
| Document Period End Date | Aug. 31, 2024 | |
| SMASh Series M Fund | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset SMASh Series M Fund | |
| Class Name | SMASh Series M Fund | |
| No Trading Symbol Flag | true | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about Western Asset SMASh Series M Fund for the period March 1, 2024, to August 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents . You can also request this information by contacting us at 877-6LM-FUND/656-3863 . | |
| Additional Information Phone Number | 877-6LM-FUND/656-3863 | |
| Additional Information Website | https://www.franklintempleton.com/regulatory-fund-documents | |
| Expenses [Text Block] | WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment) Fund Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment * , † Western Asset SMASh Series M Fund 1 $0 0.00% | [1],[2],[3] |
| Expenses Paid, Amount | $ 0 | [1] |
| Expense Ratio, Percent | 0% | [1] |
| Net Assets | $ 1,244,832,648 | |
| Holdings Count | $ / shares | 375 | [4] |
| Investment Company Portfolio Turnover | 83% | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $1,244,832,648 Total Number of Portfolio Holdings * 375 Portfolio Turnover Rate 83% | [4] |
| Holdings [Text Block] | ||
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-fund-documents | |
| [1] 1 Does not reflect the effect of fees and expenses associated with a separately managed account, or a management fee or other operating expenses of the Fund. * Reflects fee waivers and/or expense reimbursements, without which expenses would have been higher. † Annualized. * Does not include derivatives, except purchased options, if any. | ||
| Label | Element | Value |
| SMASh Series M Fund | Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 38% |
| SMASh Series M Fund | Collateralized Mortgage Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 34% |
| SMASh Series M Fund | Asset-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 10.20% |
| SMASh Series M Fund | Corporate Bonds & Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.40% |
| SMASh Series M Fund | U.S. Government & Agency Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 2.60% |
| SMASh Series M Fund | Sovereign Bonds [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.70% |
| SMASh Series M Fund | Short-Term Investment [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 11.10% |
