Share-Based Compensation - Summary of Stock Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Options
Outstanding at beginning of period (in shares)	1,261
Exercised (in shares)	(677)
Canceled (in shares)	(50)
Outstanding at end of period (in shares)	534	1,261
Exercisable (in shares)	534
Vested (in shares)	534
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 15.72
Exercised (in dollars per share)	16.02
Canceled (in dollars per share)	34.67
Outstanding at end of period (in dollars per share)	13.55	$ 15.72
Exercisable (in dollars per share)	13.55
Vested (in dollars per share)	$ 13.55
Options Activity, Weighted Remaining Contractual Term and Aggregate Intrisic Value
Outstanding weighted average remaining contractual term	1 year 10 months 20 days	2 years 5 months 22 days
Exercisable weighted average remaining contractual term	1 year 10 months 20 days
Vested weighted average remaining contractual term	1 year 10 months 20 days
Outstanding aggregate intrinsic value	$ 16	$ 28
Exercisable aggregate intrinsic value	16
Vested aggregate intrinsic value	16
Employee stock option
Options Activity, Weighted Remaining Contractual Term and Aggregate Intrisic Value
Intrinsic value of stock options exercised	17	7	$ 4
Cash received from stock options exercised	11	14	4
Fair value of stock options vested	$ 0	$ 0	$ 1