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Debt
 3 Months Ended
Mar. 31, 2024
Debt Disclosure [Abstract]  
Debt Debt
MPLX’s outstanding borrowings consist of the following:
(In millions)March 31,
2024		December 31,
2023
MPLX LP:
MPLX Credit Agreement$— $— 
Fixed rate senior notes20,657 20,657 
Consolidated subsidiaries:
MarkWest12 12 
ANDX31 31 
Finance lease obligations6 6 
Total20,706 20,706 
Unamortized debt issuance costs(119)(122)
Unamortized discount(143)(153)
Amounts due within one year(1,639)(1,135)
Total long-term debt due after one year$18,805 $19,296 
Credit Agreement
MPLX’s credit agreement (the “MPLX Credit Agreement”) matures in July 2027 and, among other things, provides for a $2 billion unsecured revolving credit facility and letter of credit issuing capacity under the facility of up to $150 million. Letter of credit issuing capacity is included in, not in addition to, the $2 billion borrowing capacity. Borrowings under the MPLX Credit Agreement bear interest, at MPLX’s election, at either the Adjusted Term SOFR or the Alternate Base Rate, both as defined in the MPLX Credit Agreement, plus an applicable margin.
There was no activity on the MPLX Credit Agreement during the three months ended March 31, 2024.
Fixed Rate Senior Notes
MPLX’s senior notes, including those issued by consolidated subsidiaries, consist of various series of senior notes maturing between 2024 and 2058 with interest rates ranging from 1.750 percent to 5.650 percent. Interest on each series of notes is payable semi-annually in arrears on various dates depending on the series of the notes.