Operating Revenues (Deferred Revenue) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Revenue from External Customer [Line Items]
Total deferred revenues	$ 43	$ 42
Lease revenues
Revenue from External Customer [Line Items]
Total deferred revenues	41	41
Service revenues
Revenue from External Customer [Line Items]
Total deferred revenues	$ 2	$ 1