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Asset Retirement Obligations (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Carrying amount of asset retirement obligations at January 1 $ 2,053 $ 2,022
Liabilities incurred 104 119
Property dispositions (108) (70)
Liabilities settled (298) (443)
Accretion expense 102 93
Revisions in estimated liabilities 206 332
Carrying amount of asset retirement obligations at December 31 $ 2,059 $ 2,053