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Related Party Agreements and Transactions (Summary of Expenses by Income Statement RP Line Item) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Related Party Transaction [Line Items] |  |  |
General and administrative expenses | $ 446 | $ 427 | $ 379
Marathon Petroleum Corporation [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
General and administrative expenses | 299 | 289 | 262
Construction-in-progress | Marathon Petroleum Corporation [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Property, Plant and Equipment, Additions | $ 230 | $ 182 | $ 94

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