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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Beginning asset retirement obligations $ 281 $ 187 $ 285 $ 189  
AssetRetirementObligationLiabilitiesAcquiredInBusinessCombination 3 0 3 0  
Asset Retirement Obligation, Liabilities Incurred 0 1 0 2  
Liabilities settled (8) (9) (21) (18)  
Accretion of discount from continuing operations 5 3 14 9  
Ending asset retirement obligations 281 $ 182 281 $ 182  
Asset retirement obligations, current portions $ 40   $ 40   $ 40