v3.26.1
Shareholder Report
 6 Months Ended
Feb. 28, 2026
USD ($)
$ / shares
Shareholder Report [Line Items]  
Document Type N-CSRS
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 Feb. 28, 2026
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Capital Shares  
Shareholder Report [Line Items]  
Fund Name Western Asset Premier Institutional Government Reserves
Class Name Capital Shares
Trading Symbol WABXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Western Asset Premier Institutional Government Reserves for the period September 1, 2025, to February 28, 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 SIX MONTHS? (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 Shares1
$6
0.11%
[1],[2],[3]
Expenses Paid, Amount $ 6 [1]
Expense Ratio, Percent 0.11% [1]
Net Assets $ 8,473,315,720
Holdings Count | $ / shares 104
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of February 28, 2026)
Total Net Assets
$8,473,315,720
Total Number of Portfolio Holdings (reflects holdings of Government Portfolio)
104
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of February 28, 2026)
Portfolio Composition*  (% of Total Investments)
image
 [4]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Premium Shares  
Shareholder Report [Line Items]  
Fund Name Western Asset Premier Institutional Government Reserves
Class Name Premium Shares
Trading Symbol WACXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Western Asset Premier Institutional Government Reserves for the period September 1, 2025, to February 28, 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 SIX MONTHS? (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*,†
Premium Shares1
$7
0.13%
[5],[6],[7]
Expenses Paid, Amount $ 7 [5]
Expense Ratio, Percent 0.13% [5]
Net Assets $ 8,473,315,720
Holdings Count | $ / shares 104
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of February 28, 2026)
Total Net Assets
$8,473,315,720
Total Number of Portfolio Holdings (reflects holdings of Government Portfolio)
104
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of February 28, 2026)
Portfolio Composition*  (% of Total Investments)
image
 [8]
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]
† Annualized.
[4]
* Reflects holdings of Government Portfolio.
[5]
1 The expense table reflects the expenses of both the feeder Fund and the master Fund.
[6]
* 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.
[7]
† Annualized.
[8]
* Reflects holdings of Government Portfolio.