Operating Revenues (Operating Revenues) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 265	$ 270	$ 773	$ 835
Lease revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	209	217	622	688
Service revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 56	$ 53	$ 151	$ 147