Revenues (Movement In Deferred Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues[Abstract]
Balance at beginning of period	$ 92	$ 91
Deferral of revenue	31	38
Recognition of deferred revenue	(31)	(33)
Balance at end of period	$ 93	$ 96