Acquisitions and Investments (Fair Value of Consideration Transferred - MarkWest) (Details) - USD ($) $ in Millions		1 Months Ended		12 Months Ended
	Dec. 04, 2015	Jul. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Business Acquisition [Line Items]
Fair value of MPLX units issued				$ 0	$ 7,326	[1]	$ 0
Payable to seller					$ 50
MarkWest
Business Acquisition [Line Items]
Fair value of MPLX units issued	$ 7,326
Cash payment to MarkWest unitholders	1,230		$ 1,280
Payable to seller	50	$ 25
Total fair value of consideration transferred	$ 8,606

[1] See Note 5.