Operating Revenues (Operating Revenues) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 241	$ 263	$ 508	$ 565
Lease revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	195	214	413	471
Service revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 46	$ 49	$ 95	$ 94