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Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Feb. 10, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total	$ 7,108		$ 4,858
Unamortized debt issuance costs	(28)		(7)
Unamortized discount	(413)		(428)
Amounts due within one year	(1)		(1)
Total long-term debt due after one year	6,666		4,422
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020	8		8
Bank revolving credit facility due 2020 | Line of Credit [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	0		0
Term loan facility due 2019 | Unsecured Debt [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	250		250
5.500% senior notes due February 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	710		710
4.500% senior notes due July 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	989		989
4.875% senior notes due December 2024 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,149		1,149
4.000% senior notes due February 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	500		500
4.875% senior notes due June 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,189		1,189
4.125% senior notes due 2027 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,250	$ 1,250	0
5.200% senior notes due 2047 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,000	$ 1,000	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

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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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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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