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Property, Plant and Equipment (Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 357 $ 247
Additional liabilities incurred 1 12
Asset Retirement Obligation, Other [Line Items] (10) 1
Liabilities settled (21) (4)
Accretion expense 16 15
Change in estimated cash flows 25 86
Ending balance $ 368 $ 357