Asset Retirement Obligations and Accrued Environmental Costs (Summary of Asset Retirement Obligations and Accrued Environmental Costs) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation and Accrual for Environmental Cost Disclosure [Abstract]
Asset retirement obligations	$ 280	$ 261	$ 268
Accrued environmental costs	441	447
Total asset retirement obligations and accrued environmental costs	721	708
Asset retirement obligations and accrued environmental costs due within one year	(83)	(84)
Long-term asset retirement obligations and accrued environmental costs	$ 638	$ 624