Debt (Narrative) (Detail) (USD $)		12 Months Ended					12 Months Ended					12 Months Ended
	Dec. 31, 2013 Trade Receivables Securitization [Member]	Dec. 31, 2013 MPC Revolver [Member] Revolving Credit Facility [Member] Period	Dec. 31, 2013 MPC Revolver [Member] Maximum [Member] Bridge Loan [Member]	Dec. 31, 2013 MPC Revolver [Member] Maximum [Member] Revolving Credit Facility [Member]	Dec. 31, 2013 MPC Revolver [Member] Maximum [Member] Letter of Credit [Member]	Dec. 31, 2013 MPC Revolver [Member] Minimum [Member] Revolving Credit Facility [Member]	Dec. 31, 2013 MPLX Revolver [Member] MPLX Operations LLC [Member] Revolving Credit Facility [Member] Period	Dec. 31, 2013 MPLX Revolver [Member] Maximum [Member] MPLX Operations LLC [Member] Bridge Loan [Member]	Dec. 31, 2013 MPLX Revolver [Member] Maximum [Member] MPLX Operations LLC [Member] Revolving Credit Facility [Member] Period	Dec. 31, 2013 MPLX Revolver [Member] Maximum [Member] MPLX Operations LLC [Member] Letter of Credit [Member]	Dec. 31, 2013 MPLX Revolver [Member] Minimum [Member] MPLX Operations LLC [Member] Revolving Credit Facility [Member]	Dec. 31, 2013 Trade Receivables Securitization [Member]	Dec. 18, 2013 Trade Receivables Securitization [Member]	Dec. 18, 2013 Trade Receivables Securitization [Member] Letter of Credit [Member]	Dec. 18, 2013 Trade Receivables Securitization [Member] Secured Debt [Member]	Dec. 31, 2013 Trade Receivables Securitization [Member] Consolidated Subsidiaries [Member]	Dec. 31, 2012 Trade Receivables Securitization [Member] Consolidated Subsidiaries [Member]
Line of Credit Facility [Line Items]
Trade receivables securitization facility due 2016	$ 0															$ 0	$ 0
Line of Credit Facility, Current Borrowing Capacity			100,000,000	2,500,000,000	2,000,000,000			50,000,000	500,000,000	250,000,000
Line of Credit Facility, Maximum Borrowing Capacity													1,300,000,000	1,250,000,000	1,300,000,000
Line Of Credit Facility Additional Borrowing Capacity				$ 500,000,000					$ 300,000,000
Number Of Renewal Periods		2					2
Line Of Credit Facility Duration Of Renewal Period		1 year					1 year
Lender Holding Percentage Of Outstanding Commitments Needed To Approve Increase In Borrowing Capacity						50.00%					50.00%
Ratio of Indebtedness to Net Capital				0.65
Number Of Prior Quarterly Reporting Periods Covenant									4
Covenant Ratio Debt To Ebitda									5.0
Covenant Ratio Debt To Ebitda Post Acquisition									5.5
Line of Credit Facility, Expiration Date												3 years