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 Master Portfolio Trust
Entity Central Index Key 0001140869
Entity Investment Company Type N-1A
Document Period End Date Aug. 31, 2026
Shareholder Report Annual or Semi-Annual annual shareholder report
U.S. Treasury Obligations Portfolio  
Shareholder Report [Line Items]  
Fund Name U.S. Treasury Obligations Portfolio
Class Name U.S. Treasury Obligations Portfolio
No Trading Symbol true
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about U.S. Treasury Obligations Portfolio 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-masterfunds-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-masterfunds-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST YEAR? (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*
U.S. Treasury Obligations Portfolio
$3
0.03%
[1]
Expenses Paid, Amount $ 3
Expense Ratio, Percent 0.03%
Net Assets $ 912,398,729
Holdings Count | $ / shares 29
Advisory Fees Paid, Amount $ 0
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$912,398,729
Total Number of Portfolio Holdings
29
Total Management Fee Paid
$0
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-masterfunds-documents
[1]
* Expense amounts may be voluntarily waived and/or reimbursed from time to time.