Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
MPLX Revolver | MPLX Operations LLC | Letter of Credit
Line of Credit Facility [Line Items]
Borrowings outstanding	$ 0
Line of Credit | MPC Revolver
Line of Credit Facility [Line Items]
Long-term debt, gross	0	0
Line of credit facility, expiration date	Sep. 14, 2017
Line of Credit | MPLX Revolver | MPLX Operations LLC
Line of Credit Facility [Line Items]
Long-term debt, gross	255	0
Line of credit facility, borrowings during period	270
Interest rate during period	1.50%
Line of credit facility, repayments during period	15
Borrowings outstanding	255
Remaining borrowing capacity	$ 245
Remaining borrowing capacity, percentage	49.00%
Line of credit facility, expiration date	Oct. 31, 2017