Assets 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 Obligations and Accrued Environmental Costs [Abstract]
Asset retirement obligations	$ 11	$ 11	$ 10
Accrued environmental costs	3	2
Total asset retirement obligations and accrued environmental costs	14	13
Asset retirement obligations and accrued environmental costs due within one year	(1)	(1)
Long-term asset retirement obligations and accrued environmental costs	$ 13	$ 12