Revenues (Movement In Deferred Revenue) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Revenues[Abstract]
Balance at beginning of period	$ 92	$ 91
Deferral of revenue	89	104
Recognition of deferred revenue	(89)	(99)
Balance at end of period	$ 92	$ 96