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Asset Retirement Obligation - Asset Retirement Obligation (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]    
Asset retirement obligation at beginning of year $ 3,085 $ 3,222
Liabilities incurred 68 171
Liabilities divested (623) (471)
Liabilities settled (90) (146)
Accretion expense 158 181
Revisions in estimated liabilities   128
Asset retirement obligation at end of year 2,598 3,085
Less current portion (36) (37)
Asset retirement obligation, long-term $ 2,562 $ 3,048