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Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Aug. 11, 2025	Feb. 18, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
MPLX Credit Agreement	$ 0			$ 0
Finance lease obligations	7			6
Debt and Lease Obligation	26,007			21,206
Unamortized debt issuance costs	(179)			(126)
Unamortized discount	(182)			(132)
Amounts due within one year	1,501			1,693
Total long-term debt due after one year	24,145			19,255
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 4,500
Senior Notes Due February 2025 \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			4.00%
MPLX LP [Member]
Debt Instrument [Line Items]
Senior Notes	25,969			21,158
MarkWest [Member]
Debt Instrument [Line Items]
Senior Notes	0			11
ANDX LP [Member]
Debt Instrument [Line Items]
Senior Notes	$ 31			$ 31