| Form N-CSR Cover | 12 Months Ended |
| Aug. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| 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 | 12 Months Ended | |
| Aug. 31, 2024 USD ($) $ / shares | ||
| Shareholder Report [Line Items] | ||
| Document Type | N-CSR | |
| 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 | |
| Capital Shares | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset Premier Institutional Liquid Reserves | |
| Class Name | Capital Shares | |
| Trading Symbol | WAAXX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Western Asset Premier Institutional Liquid Reserves for the period September 1, 2023, to August 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | 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 . | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| 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 YEAR? (based on a hypothetical $10,000 investment) Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment * Capital Shares 1 $12 0.12% | [1],[2] |
| Expenses Paid, Amount | $ 12 | [1] |
| Expense Ratio, Percent | 0.12% | [1] |
| Factors Affecting Performance [Text Block] | HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE? As of August 31, 2024, the seven-day current yield for Capital Shares of Western Asset Premier Institutional Liquid Reserves was 5.30% and the seven-day effective yield was 5.44%. The seven-day current yield represents net interest income generated by the Fund’s investments for the past seven days and assumes income is generated each week over a 365-day period. The seven-day effective yield assumes reinvestment of the coupon (interest payments) and will typically be slightly higher than the current yield because of the compounding effect on investment returns. The Fund’s performance was positively impacted by its extended weighted average maturity (WAM). This positioned the Fund well as the U.S. Federal Reserve indicated a possible shift towards a more accommodative policy stance later in the reporting period. The Fund’s floating rate holdings also provided additional yield. | |
| Performance Past Does Not Indicate Future [Text] | The Fund’s past performance is not necessarily an indication of how the Fund will perform in the future. | |
| Material Change Date | Sep. 01, 2023 | |
| Updated Performance Information Location [Text Block] | For current month-end performance, please call Franklin Templeton at 877-6LM-FUND/656-3863 or visit https://www.franklintempleton.com/investments/options/money-market-funds . | |
| Net Assets | $ 53,101,641 | |
| Holdings Count | $ / shares | 73 | [3] |
| Advisory Fees Paid, Amount | $ 103,401 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $53,101,641 Total Number of Portfolio Holdings * 73 Total Management Fee Paid $103,401 | [3] |
| Holdings [Text Block] | ||
| Material Fund Change [Text Block] | HOW HAS THE FUND CHANGED? Effective October 2, 2023, pursuant to amendments to Rule 2a-7 under the Investment Company Act of 1940, as amended (“Rule 2a-7”), the Fund may no longer impose a redemption gate and the application of liquidity fees, if any, is no longer tied to the Fund’s weekly liquid assets. Effective October 2, 2024, pursuant to Rule 2a-7, if Liquid Reserves Portfolio has total daily net redemptions that exceed 5% of the Portfolio’s net assets, or such smaller amount of net redemptions as the Board of Trustees of the Portfolio determines, based on flow information available within a reasonable period after the last computation of the Portfolio’s net asset value on that day, Liquid Reserves Portfolio may be required to apply a liquidity fee to all shares that are redeemed at a price computed on that day. In the event that Liquid Reserves Portfolio applies a liquidity fee to redemptions on a particular day, the Fund would be required, on that day, to pass the fee through to its redeeming investors on the same terms and conditions as imposed by Liquid Reserves Portfolio. This is a summary of certain changes to the Fund since September 1, 2023 . For more complete information, you may review the Fund’s current prospectus and any applicable supplements and the Fund’s next prospectus, which we expect to be available by January 1, 2025, at https://www.franklintempleton.com/regulatory-fund-documents or upon request at 877-6LM-FUND/656-3863 or prospectus franklintempleton.com . | |
| Updated Prospectus Phone Number | 877-6LM-FUND/656-3863 | |
| Updated Prospectus Email Address | prospectus@franklintempleton.com | |
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-fund-documents | |
| [1] 1 The expense table reflects the expenses of both the feeder Fund and the master Fund. * Reflects fee waivers and/or expense reimbursements, without which expenses would have been higher. Additional amounts may be voluntarily waived and/or reimbursed from time to time. * Reflects holdings of Liquid Reserves Portfolio. | ||
| Label | Element | Value |
| Capital Shares | Commercial Paper [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 46.40% |
| Capital Shares | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 19.80% |
| Capital Shares | Certificates of Deposit [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.80% |
| Capital Shares | Repurchase Agreements [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 9.80% |
| Capital Shares | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 5.70% |
| Capital Shares | Time Deposits [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.80% |
| Capital Shares | U.S. Government Agencies [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.70% |
