v3.26.3
Shareholder Report
 6 Months Ended
Aug. 31, 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 Aug. 31, 2026
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Western Asset SMASh Series TF Fund  
Shareholder Report [Line Items]  
Fund Name Western Asset SMASh Series TF Fund
Class Name Western Asset SMASh Series TF 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 TF Fund for the period  March 1, 2026, 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 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 TF Fund1
$2
0.04%
[1],[2],[3]
Expenses Paid, Amount $ 2 [1]
Expense Ratio, Percent 0.04% [1]
Net Assets $ 87,702,537
Holdings Count | $ / shares 234
Investment Company Portfolio Turnover 11.00%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$87,702,537
Total Number of Portfolio Holdings
234
Portfolio Turnover Rate
11%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
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 and does reflect the impact of interest expense related to tender option bond transactions accounted for as secured borrowings for financial reporting purposes.
[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.