v3.25.3
Shareholder Report
 12 Months Ended
Aug. 31, 2025
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, 2025
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, 2024, to August 31, 2025.
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 1-800-625-4554, or 1-203-703-6002.
Additional Information Phone Number 1-800-625-4554, or 1-203-703-6002
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 L1
$18
0.18%
[1],[2]
Expenses Paid, Amount $ 18 [1]
Expense Ratio, Percent 0.18% [1]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE?
As of August 31, 2025, the seven-day current yield for Class L shares of the Western Asset Institutional Government Reserves was 4.20% and the seven-day effective yield was 4.28%. 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 was positively impacted by its focus on the short-term supply/demand dynamics of the U.S. Treasury bill and government agency market. The anticipation of increasing deficits led to higher yield levels and offered more attractive opportunities to extend the average maturity of the Fund. The Fund also maintained a sizeable percentage in agency floating rate securities, which contributed additional yield without additional maturity risk.
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 1-800-625-4554, or 1-203-703-6002 or visit
https://www.franklintempleton.com/investments/options/money-market-funds.
Net Assets $ 14,157,264,126
Holdings Count | $ / shares 124
Advisory Fees Paid, Amount $ 19,177,955
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2025)
Total Net Assets
$14,157,264,126
Total Number of Portfolio Holdings (reflects holdings of Government Portfolio)
124
Total Management Fee Paid
$19,177,955
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2025)
Portfolio Composition* (% of Total Investments)
image
 [3]
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, 2024, to August 31, 2025.
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 1-800-625-4554, or 1-203-703-6002.
Additional Information Phone Number 1-800-625-4554, or 1-203-703-6002
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 Shares1
$16
0.16%
[4],[5]
Expenses Paid, Amount $ 16 [4]
Expense Ratio, Percent 0.16% [4]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE?
As of August 31, 2025, the seven-day current yield for Institutional Shares of the Western Asset Institutional Government Reserves was 4.22% and the seven-day effective yield was 4.31%. 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 was positively impacted by its focus on the short-term supply/demand dynamics of the U.S. Treasury bill and government agency market. The anticipation of increasing deficits led to higher yield levels and offered more attractive opportunities to extend the average maturity of the Fund. The Fund also maintained a sizeable percentage in agency floating rate securities, which contributed additional yield without additional maturity risk.
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 1-800-625-4554, or 1-203-703-6002 or visit
https://www.franklintempleton.com/investments/options/money-market-funds.
Net Assets $ 14,157,264,126
Holdings Count | $ / shares 124
Advisory Fees Paid, Amount $ 19,177,955
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2025)
Total Net Assets
$14,157,264,126
Total Number of Portfolio Holdings (reflects holdings of Government Portfolio)
124
Total Management Fee Paid
$19,177,955
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2025)
Portfolio Composition* (% of Total Investments)
image
 [6]
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, 2024, to August 31, 2025.
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 1-800-625-4554, or 1-203-703-6002.
Additional Information Phone Number 1-800-625-4554, or 1-203-703-6002
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 Shares1
$21
0.21%
[7],[8]
Expenses Paid, Amount $ 21 [7]
Expense Ratio, Percent 0.21% [7]
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE?
As of August 31, 2025, the seven-day current yield for Investor Shares of the Western Asset Institutional Government Reserves was 4.17% and the seven-day effective yield was 4.25%. 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 was positively impacted by its focus on the short-term supply/demand dynamics of the U.S. Treasury bill and government agency market. The anticipation of increasing deficits led to higher yield levels and offered more attractive opportunities to extend the average maturity of the Fund. The Fund also maintained a sizeable percentage in agency floating rate securities, which contributed additional yield without additional maturity risk.
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 1-800-625-4554, or 1-203-703-6002 or visit
https://www.franklintempleton.com/investments/options/money-market-funds.
Net Assets $ 14,157,264,126
Holdings Count | $ / shares 124
Advisory Fees Paid, Amount $ 19,177,955
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2025)
Total Net Assets
$14,157,264,126
Total Number of Portfolio Holdings (reflects holdings of Government Portfolio)
124
Total Management Fee Paid
$19,177,955
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2025)
Portfolio Composition* (% of Total Investments)
image
 [9]
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.
[2]
* 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.
[3]
* Reflects holdings of Government Portfolio.
[4]
1 The expense table reflects the expenses of both the feeder Fund and the master Fund.
[5]
* 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.
[6]
* Reflects holdings of Government Portfolio.
[7]
1 The expense table reflects the expenses of both the feeder Fund and the master Fund.
[8]
* 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.
[9]
* Reflects holdings of Government Portfolio.