Stock-Based Compensation - Activity, Pricing and Other Information for Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options Outstanding, Beginning Balance (in shares)	9,882,617
Granted, Number of Options (in shares)	346,088
Exercised, Number of Options (in shares)	(1,960,444)
Lapsed or canceled, Number of Options (in shares)	(144,417)
Number of Options Outstanding, Ending Balance (in shares)	8,123,844	9,882,617
Expected to vest, Number of Options (in shares)	2,508,684
Number of Options Exercisable, Ending Balance (in shares)	5,479,638
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price Per Option Outstanding, Beginning Balance (in dollars per share)	$ 11.28
Granted, Weighted-Average Exercise Price Per Option (in dollars per share)	21.02
Exercised, Weighted-Average Exercise Price Per Option (in dollars per share)	10.12
Lapsed or canceled, Weighted-Average Exercise Price Per Option (in dollars per share)	14.37
Weighted-Average Exercise Price Per Option Outstanding, Ending Balance (in dollars per share)	11.92	$ 11.28
Expected to vest, Weighted-Average Exercise Price Per Option (in dollars per share)	15.09
Weighted-Average Exercise Price Per Option Exercisable, Ending Balance (in dollars per share)	$ 10.36
Weighted-Average Remaining Life (Years) Outstanding	5 years 3 months 18 days	5 years 6 months
Expected to vest, Weighted-Average Remaining Life (Years)	7 years 7 months 6 days
Weighted-Average Remaining Life (Years) Exercisable	4 years 2 months 12 days
Aggregate Intrinsic Value Outstanding	$ 31	$ 88
Expected to vest, Aggregate Intrinsic Value	5
Aggregate Intrinsic Value Exercisable	$ 26