| Form N-CSR Cover | 12 Months Ended |
| Aug. 31, 2024 | |
| 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, 2024 |
| Shareholder Report | 12 Months Ended | |
| Aug. 31, 2024 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, 2024 | |
| U.S. Treasury Reserves Portfolio | ||
| Shareholder Report [Line Items] | ||
| Fund Name | U.S. Treasury Reserves Portfolio | |
| Class Name | U.S. Treasury Reserves Portfolio | |
| No Trading Symbol Flag | true | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about U.S. Treasury Reserves Portfolio for the period September 1, 2023, to August 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | annual shareholder report | |
| 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 877-6LM-FUND/656-3863 . | |
| Additional Information Phone Number | 877-6LM-FUND/656-3863 | |
| 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 Reserves Portfolio $1 0.01% | [1] |
| Expenses Paid, Amount | $ 1 | |
| Expense Ratio, Percent | 0.01% | |
| Net Assets | $ 17,011,484,407 | |
| Holdings Count | $ / shares | 59 | |
| Advisory Fees Paid, Amount | $ 0 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $17,011,484,407 Total Number of Portfolio Holdings 59 Total Management Fee Paid $0 | |
| Holdings [Text Block] | ||
| Updated Prospectus Web Address | https://www.franklintempleton.com/regulatory-masterfunds-documents | |
| [1] * 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. | ||
| Label | Element | Value |
| U.S. Treasury Reserves Portfolio | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 85.70% |
| U.S. Treasury Reserves Portfolio | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 14.30% |
