v3.26.1
Shareholder Report
 6 Months Ended
Jun. 30, 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 Jun. 30, 2026
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
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 semi-annual shareholder report contains important information about Western Asset SMASh Series Core Plus Completion Fund for the period  January 1, 2026, to June 30, 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 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 Plus Completion Fund1
$0
0.00%
[1],[2],[3]
Expenses Paid, Amount $ 0 [1]
Expense Ratio, Percent 0.00% [1]
Net Assets $ 607,862,516
Holdings Count | $ / shares 314
Investment Company Portfolio Turnover 287.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of June 30, 2026)
Total Net Assets
$607,862,516
Total Number of Portfolio Holdings (excludes derivatives, except purchased options, if any)
314
Portfolio Turnover Rate
287%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of June 30, 2026)
Portfolio Composition*,† (% of Total Investments)
image
 [4],[5]
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.  
[5]
† Certain categories may represent less than 0.1%.