Fair Value Measurements (Reconciliation Of Net Beginning And Ending Balances Recorded For Net Assets And Liabilities Classified As Level 3) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 2,402		$ 865
Total realized and unrealized losses included in net income	(2)			(1)
Purchases of PFD Preferred Stock		10,326	[1]	9,709	[1]
Redemptions of PFD Preferred Stock		(12,730)	[1]	(8,019)	[1]
Settlements of derivative instruments	2	2		(2)
Distributions to Marathon Oil				(150)	[2]
Ending balance				$ 2,402

[1] For information on PFD Preferred Stock, see Note 5. The fair value of our PFD Preferred Stock investment was measured using an income approach since the securities were not publicly traded; therefore, they were classified as Level 3 in the fair value hierarchy.

[2] Due to the January 1, 2010 merger of two non-operating RM&T Business legal entities into Marathon Oil.