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Asset Retirement Obligations and Accrued Environmental Costs (Summary of Asset Retirement Obligations and Accrued Environmental Costs) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation and Accrual for Environmental Cost Disclosure [Abstract]      
Asset retirement obligations $ 244 $ 251 $ 279
Accrued environmental costs 496 485  
Total asset retirement obligations and accrued environmental costs 740 736  
Asset retirement obligations and accrued environmental costs due within one year (85) (71)  
Long-term asset retirement obligations and accrued environmental costs $ 655 $ 665