Share-Based Compensation - Summary of Stock Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Options
Outstanding at beginning of period (in shares)	2,174
Exercised (in shares)	(590)
Canceled (in shares)	(323)
Outstanding at end of period (in shares)	1,261	2,174
Exercisable (in shares)	1,261
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 22.55
Exercised (in dollars per share)	23.88
Canceled (in dollars per share)	46.80
Outstanding at end of period (in dollars per share)	15.72	$ 22.55
Exercisable (in dollars per share)	$ 15.72
Options Activity, Weighted Remaining Contractual Term and Aggregate Intrisic Value
Outstanding weighted average remaining contractual term (years)	2 years 5 months 23 days	2 years 9 months 10 days
Exercisable weighted average remaining contractual term (years)	2 years 5 months 23 days
Outstanding aggregate intrinsic value	$ 28	$ 22
Exercisable aggregate intrinsic value	$ 28
Vested or expected to vest
Number of options	1,261
Weighted average exercise price (in dollars per share)	$ 15.72
Weighted Average Remaining Contractual Term (years)	2 years 5 months 23 days
Aggregate intrinsic value	$ 28
Employee stock option
Vested or expected to vest
Intrinsic value of stock options exercised	7	4	$ 6
Cash received from stock options exercised	14	4	6
Fair value of stock options vested	$ 0	$ 1	$ 1