MPLX LP (Narrative) (Detail) - MPLX LP		12 Months Ended
	Dec. 04, 2015	Dec. 31, 2016 Barge Tow_Boat
Noncontrolling Interest [Line Items]
Number of tow boats | Tow_Boat		18
Number of barges | Barge		200
MPC's ownership percentage of the general partner interest		100.00%
Noncontrolling interest (in percentage)		74.50%
MarkWest
Noncontrolling Interest [Line Items]
Common units conversion ratio	1.09
Butane Cavern
Noncontrolling Interest [Line Items]
Ownership interest (in percentage)		100.00%
General Partner
Noncontrolling Interest [Line Items]
MPC's partnership interest in MPLX (in percentage)		2.00%
General Partner and Limited Partner
Noncontrolling Interest [Line Items]
MPC's partnership interest in MPLX (in percentage)		25.50%
MPLX Pipe Line Holdings LP
Noncontrolling Interest [Line Items]
Ownership interest (in percentage)	100.00%	100.00%