Stock-Based Compensation (Summary Of Stock Option Award Activity) (Details) - Stock Options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercised	$ 25	$ 23	$ 44
Unrecognized compensation cost	$ 15
Weighted average recognition period, in years	1 year 9 months 18 days
Number of  Shares
Outstanding, beginning balance	10,018,367
Granted	2,770,139
Exercised	(1,016,593)
Forfeited or expired	(472,132)
Outstanding, ending balance	11,299,781	10,018,367
Vested and expected to vest at December 31, 2020 (in shares)	11,243,905
Exercisable at December 31, 2020 (in shares)	7,641,774
Weighted Average Exercise Price
Outstanding, beginning balance (in USD per share)	$ 42.55
Granted (in USD per share)	28.78	$ 61.92	$ 67.71
Exercised (in USD per share)	14.15
Forfeited or expired (in USD per share)	37.77
Outstanding, ending balance (in USD per share)	41.95	$ 42.55
Vested and expected to vest at December 31, 2020 (in USD per share)	29.28
Exercisable at December 31, 2020 (in USD per share)	$ 42.61
Weighted Average Remaining Contractual Terms (in years)
Vested and expected to vest at December 31, 2020 (in years)	5 years 2 months 12 days
Exercisable at December 31, 2020 (in years)	3 years 7 months 6 days
Aggregate Intrinsic Value (in millions)
Vested and expected to vest at December 31, 2020 (in USD)	$ 76
Exercisable at December 31, 2020 (in USD)	$ 42