v3.26.3
Shareholder Report
 12 Months Ended
Aug. 31, 2026
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, 2026
Shareholder Report Annual or Semi-Annual annual shareholder report
Select Shares  
Shareholder Report [Line Items]  
Fund Name Western Asset Select Tax Free Reserves
Class Name Select Shares
Trading Symbol CIFXX
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Western Asset Select Tax Free Reserves for the period September 1, 2025, to August 31, 2026.
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*
Select Shares
$18
0.18%
[1]
Expenses Paid, Amount $ 18
Expense Ratio, Percent 0.18%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE?
As of August 31, 2026, the seven-day current yield for Select Shares of the Western Asset Select Tax Free Reserves was 2.34% and the seven-day effective yield was 2.37%. 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 maintained a higher-quality bias and a shorter-duration profile during the reporting period, prioritizing liquidity. This positioning contributed to performance as yields moved higher.
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 $ 122,607,096
Holdings Count | $ / shares 72
Advisory Fees Paid, Amount $ 115,203
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$122,607,096
Total Number of Portfolio Holdings
72
Total Management Fee Paid
$115,203
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition (% of Total Investments)
image
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Investor Shares  
Shareholder Report [Line Items]  
Fund Name Western Asset Select Tax Free Reserves
Class Name Investor Shares
Trading Symbol LTFXX
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Western Asset Select Tax Free Reserves for the period September 1, 2025, to August 31, 2026.
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 Shares
$23
0.23%
[2]
Expenses Paid, Amount $ 23
Expense Ratio, Percent 0.23%
Factors Affecting Performance [Text Block]
HOW DID THE FUND PERFORM LAST YEAR AND WHAT AFFECTED ITS PERFORMANCE?
As of August 31, 2026, the seven-day current yield for Investor Shares of the Western Asset Select Tax Free Reserves was 2.29% and the seven-day effective yield was 2.32%. 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 maintained a higher-quality bias and a shorter-duration profile during the reporting period, prioritizing liquidity. This positioning contributed to performance as yields moved higher.
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 $ 122,607,096
Holdings Count | $ / shares 72
Advisory Fees Paid, Amount $ 115,203
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$122,607,096
Total Number of Portfolio Holdings
72
Total Management Fee Paid
$115,203
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition (% of Total Investments)
image
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
[1]
* 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.
[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.