Asset Retirement Obligations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of asset retirement obligations at January 1	$ 1,768	[1]	$ 1,571
Liabilities incurred	70		39
Property dispositions	(78)		(4)
Liabilities settled	(89)		(68)
Accretion expense	110		100
Revisions in estimated liabilities	104		130
Carrying amount of asset retirement obligations at December 31	1,885	[1]	1,768	[1]
Table Text Block Supplement [Abstract]
Short-term asset retirement obligations	$ 298		$ 31

[1] Short-term AROs of $298 million at December 31, 2012, and $31 million at December 31, 2011, were included in accrued expenses on the Company’s Consolidated Balance Sheets.