Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		$ (36)	$ 0	$ (36)		$ 0
Depreciation and amortization	[1]	346	302	992		916
Income Tax Expense (Benefit)		(1)	(4)	(1)		(2)
Amortization of Debt Issuance Costs				(44)		(29)
Share-based Payment Arrangement, Expense		(4)	(5)	(12)		(17)
Goodwill and Intangible Asset Impairment		0	0	(2,165)		0
Interest and Other Financial Costs		(223)	(223)	(647)		(657)
Extinguishment of Debt, Gain (Loss), Net of Tax		14	0	14		0
Income/(loss) from equity method investments(1)		83	95	(1,012)	[2],[3]	255
Proceeds from Equity Method Investment, Distribution, Return of Capital				(350)		(379)
Acquisition Costs, Period Cost		0	(9)	0		(14)
Other Cost and Expense, Operating		3	1	5		1
Net income		674	689	(1,387)		2,035
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		1,335	1,165	3,856		3,015
Amortization of Debt Issuance Costs		(15)	(10)	(44)		(29)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(130)	(145)	(369)		(399)
Adjusted EBITDA attributable to noncontrolling interests		10	9	27		23
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		164	113	440		373
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		(27)	0	(27)		0
Adjusted EBITDA	[4]	893	766	2,604		1,895
Income/(loss) from equity method investments(1)		36	60	126		159
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		182	189	552		543
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Restructuring Charges		(9)	0	(9)		0
Adjusted EBITDA	[4]	442	399	1,252		1,120
Income/(loss) from equity method investments(1)		47	35	(1,138)		96
Not Designated as Hedging Instrument [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[5]	$ (10)	$ 11	$ (1)		$ 7

[1] Depreciation and amortization attributable to L&S was $164 million and $440 million for the three and nine months ended September 30, 2020, respectively, and $113 million and $373 million for the three and nine months ended September 30, 2019, respectively. Depreciation and amortization attributable to G&P was $182 million and $552 million for the three and nine months ended September 30, 2020, respectively, and $189 million and $543 million for the three and nine months ended September 30, 2019, respectively.

[2] The 2020 period includes $1,264 million of impairment expense. See Note 4.

[3] The nine months ended September 30, 2020 includes $1,264 million of impairment expense. See Note 4.

[4] See below for the reconciliation from Segment Adjusted EBITDA to net income.

[5] MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.