Asset Retirement Obligation - Asset Retirement Obligation (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement obligation at the beginning of period	$ 2,598
Liabilities incurred	7
Liabilities acquired	34
Liabilities divested	(1)
Liabilities settled	(35)
Accretion expense	116
Revisions in estimated liabilities	59
Asset retirement obligation at the end of period	2,778
Less current portion	36	$ 36
Asset retirement obligation, long-term	$ 2,742	$ 2,562	[1]

[1] Financial information for 2015 has been recast to reflect retrospective application of the successful efforts method of accounting. See Note 1.