Derivative Financial Instruments (Derivatives Income Statement Location) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains	[1]			$ (6)
Total derivative loss		$ (5)	$ (3)	(24)
Product sales
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		0	4	(9)
Unrealized gains				4
Total derivative loss		0	6	(5)
Purchased product costs
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		(6)	(12)	(9)
Unrealized gains				(10)
Total derivative loss		(5)	(9)	(19)
Cost of revenues
Derivative Instruments, Gain (Loss) [Line Items]
Realized gains/(losses)		0	0	0
Unrealized gains				0
Total derivative loss		0	0	$ 0
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	[1]	1	5
Not Designated as Hedging Instrument | Product sales
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		0	2
Not Designated as Hedging Instrument | Purchased product costs
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		1	3
Not Designated as Hedging Instrument | Cost of revenues
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		$ 0	$ 0

[1] MPLX makes a distinction between realized or 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.