v3.25.3
Shareholder Report
 6 Months Ended
Aug. 31, 2025
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 Aug. 31, 2025
Western Asset SMASh Series Core Completion Fund  
Shareholder Report [Line Items]  
Fund Name Western Asset SMASh Series Core Completion Fund
Class Name Western Asset SMASh Series Core Completion Fund
No Trading Symbol true
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Western Asset SMASh Series Core Completion Fund for the period March 1, 2025, to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-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 SIX MONTHS? (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 Completion Fund1
$0
0.00%
[1],[2],[3]
Expenses Paid, Amount $ 0 [1]
Expense Ratio, Percent 0.00% [1]
Net Assets $ 114,845,706
Holdings Count | $ / shares 198
Investment Company Portfolio Turnover 264.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2025)
Total Net Assets
$114,845,706
Total Number of Portfolio Holdings (excludes derivatives, except purchased options, if any)
198
Portfolio Turnover Rate
264%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2025)
Portfolio Composition* (% of Total Investments)
image
 [4]
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.
[2]
* Reflects fee waivers and/or expense reimbursements, without which expenses would have been higher.
[3]
† Annualized.
[4]
* Does not include derivatives, except purchased options, if any.