| 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 | |
| Liquid Reserves Portfolio | ||
| Shareholder Report [Line Items] | ||
| Fund Name | Liquid Reserves Portfolio | |
| Class Name | Liquid Reserves Portfolio | |
| No Trading Symbol Flag | true | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about Liquid 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 . | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| 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 * Liquid Reserves Portfolio $1 0.01% | [1] |
| Expenses Paid, Amount | $ 1 | |
| Expense Ratio, Percent | 0.01% | |
| Material Change Date | Sep. 01, 2023 | |
| Net Assets | $ 8,307,942,958 | |
| Holdings Count | $ / shares | 73 | |
| Advisory Fees Paid, Amount | $ 0 | |
| Additional Fund Statistics [Text Block] | KEY FUND STATISTICS (as of August 31, 2024) Total Net Assets $8,307,942,958 Total Number of Portfolio Holdings 73 Total Management Fee Paid $0 | |
| Holdings [Text Block] | ||
| Material Fund Change [Text Block] | HOW HAS THE FUND CHANGED? Effective October 2, 2023, pursuant to amendments to Rule 2a-7 under the Investment Company Act of 1940, as amended (“Rule 2a-7”), the Portfolio may no longer impose a redemption gate and the application of liquidity fees, if any, is no longer tied to the Portfolio’s weekly liquid assets. Effective October 2, 2024, pursuant to Rule 2a-7, if Liquid Reserves Portfolio has total daily net redemptions that exceed 5% of the Portfolio’s net assets, or such smaller amount of net redemptions as the Board of Trustees of the Portfolio determines, based on flow information available within a reasonable period after the last computation of the Portfolio’s net asset value on that day, Liquid Reserves Portfolio may be required to apply a liquidity fee to all shares that are redeemed at a price computed on that day. This is a summary of certain changes to the Portfolio since September 1, 2023 . | |
| 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 |
| Liquid Reserves Portfolio | Commercial Paper [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 46.40% |
| Liquid Reserves Portfolio | U.S. Treasury Bills [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 19.80% |
| Liquid Reserves Portfolio | Certificates of Deposit [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 13.80% |
| Liquid Reserves Portfolio | Repurchase Agreements [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 9.80% |
| Liquid Reserves Portfolio | U.S. Treasury Notes [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 5.70% |
| Liquid Reserves Portfolio | Time Deposits [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 3.80% |
| Liquid Reserves Portfolio | U.S. Government Agencies [Member] | ||
| Holdings [Line Items] | oef_HoldingsLineItems | |
| Percent of Total Investments | oef_PctOfTotalInv | 0.70% |
