Revenues (Movement In Deferred Revenue) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Revenues[Abstract]
Balance at beginning of period	$ 92	$ 91
Deferral of revenue	63	75
Recognition of deferred revenue	(59)	(68)
Balance at end of period	$ 95	$ 97