| 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 | |
| Class L | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset Institutional Government Reserves | |
| Class Name | Class L | |
| Trading Symbol | LWPXX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Western Asset Institutional Government 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 . | |
| 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 * Class L 1 $20 0.19% | [1],[2] |
| Expenses Paid, Amount | $ 20 | [1] |
| Expense Ratio, Percent | 0.19% | [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 Class L shares of Western Asset Institutional Government Reserves was 5.17% and the seven-day effective yield was 5.30%. 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 an extension of its weighted average maturity (WAM), and weighted average life (WAL) during the period. The maturity extension positioned the Fund well as yields fell in anticipation of a greater likelihood of the U.S. Federal Reserve moving towards a more accommodative policy stance. | |
| 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. | |
| 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 | $ 13,146,846,893 | |
| Holdings Count | $ / shares | 137 | [3] |
| Advisory Fees Paid, Amount | $ 20,074,773 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $13,146,846,893 Total Number of Portfolio Holdings * 137 Total Management Fee Paid $20,074,773 | [3] |
| Holdings [Text Block] | ||
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-fund-documents | |
| Institutional Shares | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset Institutional Government Reserves | |
| Class Name | Institutional Shares | |
| Trading Symbol | INGXX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Western Asset Institutional Government 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 . | |
| 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 * Institutional Shares 1 $17 0.17% | [4],[5] |
| Expenses Paid, Amount | $ 17 | [4] |
| Expense Ratio, Percent | 0.17% | [4] |
| 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 Institutional Shares shares of Western Asset Institutional Government Reserves was 5.19% and the seven-day effective yield was 5.32%. 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 an extension of its weighted average maturity (WAM), and weighted average life (WAL) during the period. The maturity extension positioned the Fund well as yields fell in anticipation of a greater likelihood of the U.S. Federal Reserve moving towards a more accommodative policy stance. | |
| 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. | |
| 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 | $ 13,146,846,893 | |
| Holdings Count | $ / shares | 137 | [6] |
| Advisory Fees Paid, Amount | $ 20,074,773 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $13,146,846,893 Total Number of Portfolio Holdings * 137 Total Management Fee Paid $20,074,773 | [6] |
| Holdings [Text Block] | ||
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-fund-documents | |
| Investor Shares | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset Institutional Government Reserves | |
| Class Name | Investor Shares | |
| Trading Symbol | LGRXX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Western Asset Institutional Government 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 . | |
| 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 * Investor Shares 1 $23 0.22% | [7],[8] |
| Expenses Paid, Amount | $ 23 | [7] |
| Expense Ratio, Percent | 0.22% | [7] |
| 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 Investor Shares shares of Western Asset Institutional Government Reserves was 5.14% and the seven-day effective yield was 5.27%. 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 an extension of its weighted average maturity (WAM), and weighted average life (WAL) during the period. The maturity extension positioned the Fund well as yields fell in anticipation of a greater likelihood of the U.S. Federal Reserve moving towards a more accommodative policy stance. | |
| 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. | |
| 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 | $ 13,146,846,893 | |
| Holdings Count | $ / shares | 137 | [9] |
| Advisory Fees Paid, Amount | $ 20,074,773 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $13,146,846,893 Total Number of Portfolio Holdings * 137 Total Management Fee Paid $20,074,773 | [9] |
| Holdings [Text Block] | ||
| 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 Government Portfolio. 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 Government Portfolio. 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 Government Portfolio. | ||
| Label | Element | Value |
| Class L | Repurchase Agreements [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 62.80% |
| Class L | U.S. Government Agencies [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 18.90% |
| Class L | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.60% |
| Class L | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 4.70% |
| Label | Element | Value |
| Institutional Shares | Repurchase Agreements [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 62.80% |
| Institutional Shares | U.S. Government Agencies [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 18.90% |
| Institutional Shares | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.60% |
| Institutional Shares | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 4.70% |
| Label | Element | Value |
| Investor Shares | Repurchase Agreements [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 62.80% |
| Investor Shares | U.S. Government Agencies [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 18.90% |
| Investor Shares | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.60% |
| Investor Shares | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 4.70% |
