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Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Feb. 08, 2018	Jan. 02, 2018	Dec. 31, 2017	Jul. 21, 2017	Feb. 10, 2017
Debt Instrument [Line Items]
Total	$ 12,357			$ 7,362
Unamortized debt issuance costs	(79)			(27)
Unamortized discount	(416)			(389)
Amounts due within one year	(1)			(1)
Total long-term debt due after one year	11,861			6,945
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross		$ 5,500
Capital Lease Obligations [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020	7			7
Bank revolving credit facility due 2022 | MPLX LP
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 2,250
Repayments of Long-term Lines of Credit	555
Bank revolving credit facility due 2022 | Line of Credit [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	0			505
5.500% senior notes due February 2023 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	710			710
Senior Notes Due March 2023 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500			0
4.500% senior notes due July 2023 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	989			989
4.875% senior notes due December 2024 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,149			1,149
4.000% senior notes due February 2025 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500			500
4.875% senior notes due June 2025 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,189			1,189
4.125% senior notes due March 2027 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,250			1,250		$ 1,250
Senior Notes Due March 2028 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,250			0
Senior Notes Due April 2038 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,750			0
5.200% senior notes due March 2047 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,000			1,000		$ 1,000
Senior Notes Due April 2048 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,500			0
Senior Notes Due April 2058 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500			0
MarkWest - 4.500% - 5.500% senior notes, due 2023-2025 | Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 63			$ 63
MPLX 364-Day Term Loan [Member] | MPLX LP
Debt Instrument [Line Items]
Current borrowing capacity			$ 4,100

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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