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Derivative Financial Instruments (Derivatives Income Statement Location) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized gains/(losses) [1] $ 5 $ (6) $ (36)
Total derivative loss (3) (24) (42)
Product sales      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gains/(losses) 4 (9) 2
Unrealized gains/(losses) 2 4 (15)
Total derivative loss 6 (5) (13)
Purchased product costs      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gains/(losses) (12) (9) (5)
Unrealized gains/(losses) 3 (10) (22)
Total derivative loss (9) (19) (27)
Cost of revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gains/(losses) 0 0 (3)
Unrealized gains/(losses) 0 0 1
Total derivative loss $ 0 $ 0 $ (2)
[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.