| Form N-CSR Cover | 12 Months Ended |
| Dec. 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 | Dec. 31, 2024 |
| Shareholder Report | 12 Months Ended | |
| Dec. 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 | Dec. 31, 2024 | |
| Western Asset SMASh Series Core Plus Completion Fund | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Western Asset SMASh Series Core Plus Completion Fund | |
| Class Name | Western Asset SMASh Series Core Plus Completion Fund | |
| No Trading Symbol | true | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Western Asset SMASh Series Core Plus Completion Fund for the period January 1, 2024, to December 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) Fund Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment * Western Asset SMASh Series Core Plus Completion Fund 1 $0 0.00% | [1],[2] |
| Expenses Paid, Amount | $ 0 | [1] |
| Expense Ratio, Percent | 0% | [1] |
| Factors Affecting Performance [Text Block] | HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE? For the twelve months ended December 31, 2024, Western Asset SMASh Series Core Plus Completion Fund returned -3.64%. The Fund compares its performance to the Bloomberg U.S. Aggregate Index, which returned 1.25% for the same period. PERFORMANCE HIGHLIGHTS Top contributors to performance: ↑ Structured products as spreads generally tightened ↑ High-yield credit positioning as spreads tightened ↑ Investment-grade credit positioning as spreads tightened Top detractors from performance: ↓ Emerging market positioning ↓ Long duration position as yields rose ↓ Yield curve positioning as the curve steepened Use of derivatives and the impact on performance: The Fund utilized treasury futures, options and swaps to manage duration, yield curve positioning and credit exposure. In aggregate, these derivatives detracted from performance. | |
| 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. | |
| Line Graph [Table Text Block] | ||
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS (%) Period Ended December 31, 2024 1 Year 5 Year 10 Year Western Asset SMASh Series Core Plus Completion Fund -3.64 -5.64 0.47 Bloomberg U.S. Aggregate Index 1.25 -0.33 1.35 | |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares, or the effect of fees and expenses borne by investors at the separately managed account level. | |
| Material Change Date | Aug. 21, 2024 | |
| 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/mutual-funds . Important data provider notices and terms available at www.franklintempletondatasources.com. | |
| Net Assets | $ 1,071,686,627 | |
| Holdings Count | $ / shares | 331 | [3] |
| Advisory Fees Paid, Amount | $ 0 | |
| Investment Company Portfolio Turnover | 266% | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of December 31, 2024) Total Net Assets $1,071,686,627 Total Number of Portfolio Holdings * 331 Total Management Fee Paid $0 Portfolio Turnover Rate 266% | [3] |
| Holdings [Text Block] | ||
| Material Fund Change [Text Block] | HOW HAS THE FUND CHANGED? Effective August 21, 2024 , the Fund’s named portfolio management team consists of Michael C. Buchanan, Ryan K. Brist, Kevin J. Ritter, Walter E. Kilcullen and Julien A. Scholnick. This is a summary of certain changes to the Fund since January 1, 2024. For more complete information, you may review the Fund’s current prospectus and any applicable supplements and the Fund’s next prospectus, which we expect to be available by May 1, 2025, at https://www.franklintempleton.com/regulatory-fund-documents or upon request at 877-6LM-FUND/656-3863 or prospectus franklintempleton.com . | |
| Updated Prospectus Phone Number | 877-6LM-FUND/656-3863 | |
| Updated Prospectus Email Address | prospectus@franklintempleton.com | |
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-fund-documents | |
| [1] 1 Does not reflect the effect of fees and expenses associated with a separately managed account, or a management fee or other operating expenses of the Fund. * Reflects fee waivers and/or expense reimbursements, without which expenses would have been higher. * Does not include derivatives, except purchased options, if any. | ||
| Shareholder Report, Line Graph (Details) - USD ($) | 12 Months Ended | ||||||||||
| Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | |
| Western Asset SMASh Series Core Plus Completion Fund | |||||||||||
| Account Value [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Western Asset SMASh Series Core Plus Completion Fund | ||||||||||
| Account Value | $ 10,478 | $ 10,873 | $ 10,126 | $ 14,285 | $ 14,250 | $ 14,010 | $ 11,744 | $ 12,075 | $ 11,047 | $ 10,045 | $ 10,000 |
| Bloomberg U.S. Aggregate Index [Member] | |||||||||||
| Account Value [Line Items] | |||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Index | ||||||||||
| Account Value | $ 11,432 | $ 11,291 | $ 10,699 | $ 12,299 | $ 12,492 | $ 11,620 | $ 10,688 | $ 10,687 | $ 10,321 | $ 10,055 | $ 10,000 |
| Shareholder Report, Average Annual Return (Details) | 12 Months Ended | 60 Months Ended | 120 Months Ended |
| Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2024 | |
| Western Asset SMASh Series Core Plus Completion Fund | |||
| Average Annual Return [Line Items] | |||
| Line Graph and Table Measure Name | Western Asset SMASh Series Core Plus Completion Fund | ||
| Average Annual Return, Percent | (3.64%) | (5.64%) | 0.47% |
| Bloomberg U.S. Aggregate Index [Member] | |||
| Average Annual Return [Line Items] | |||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Index | ||
| Average Annual Return, Percent | 1.25% | (0.33%) | 1.35% |
| Label | Element | Value |
| Western Asset SMASh Series Core Plus Completion Fund | Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 30.40% |
| Western Asset SMASh Series Core Plus Completion Fund | Sovereign Bonds [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.40% |
| Western Asset SMASh Series Core Plus Completion Fund | Collateralized Mortgage Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 12.60% |
| Western Asset SMASh Series Core Plus Completion Fund | U.S. Government & Agency Obligations [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 9% |
| Western Asset SMASh Series Core Plus Completion Fund | Consumer Discretionary [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 6.10% |
| Western Asset SMASh Series Core Plus Completion Fund | Energy [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 5.70% |
| Western Asset SMASh Series Core Plus Completion Fund | Health Care [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 4.10% |
| Western Asset SMASh Series Core Plus Completion Fund | Industrials [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.80% |
| Western Asset SMASh Series Core Plus Completion Fund | Communication Services [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.50% |
| Western Asset SMASh Series Core Plus Completion Fund | Financials [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 2% |
| Western Asset SMASh Series Core Plus Completion Fund | Asset-Backed Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 1.80% |
| Western Asset SMASh Series Core Plus Completion Fund | U.S. Treasury Inflation Protected Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 1.60% |
| Western Asset SMASh Series Core Plus Completion Fund | Materials [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 1.50% |
| Western Asset SMASh Series Core Plus Completion Fund | Consumer Staples [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 1.10% |
| Western Asset SMASh Series Core Plus Completion Fund | Information Technology [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.60% |
| Western Asset SMASh Series Core Plus Completion Fund | Purchased Options [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.30% |
| Western Asset SMASh Series Core Plus Completion Fund | Real Estate [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.20% |
| Western Asset SMASh Series Core Plus Completion Fund | Non-U.S. Treasury Inflation Protected Securities [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0% |
| Western Asset SMASh Series Core Plus Completion Fund | Short-Term Investments [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 2.30% |
