Stock-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Stock Options
Grant date exercise price	$ 34
Stock Options
Stock Options
Vesting period	5 years
Expiration period	10 years
Shares
Outstanding at the beginning of the period (in shares)	7,000,000	12,000,000
Granted (in shares)	397,350	0
Exercised (in shares)	0	1,000,000
Forfeited (in shares)	0	0
Outstanding at the end of the period (in shares)	7,000,000	11,000,000
Option shares vested and expected to vest at the end of the period (in shares)	7,000,000	11,000,000
Option shares exercisable at the end of the period (in shares)	6,000,000	10,000,000
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 15	$ 17
Granted (in dollars per share)	34	26
Exercised (in dollars per share)	23	18
Forfeited (in dollars per share)	0	0
Outstanding at the end of the period (in dollars per share)	16	18
Option shares vested and expected to vest at the end of the period (in dollars per share)	16	18
Option shares exercisable at the end of the period (in dollars per share)	$ 13	$ 17
Aggregate intrinsic value
Exercised	$ 3	$ 18
Option shares vested and expected to vest at the end of the period	131	154
Option shares exercisable at the end of the period	$ 120	$ 138
Weighted average remaining option term
Outstanding at the end of the period	4 years	4 years
Option shares vested and expected to vest at the end of the period	4 years	4 years
Option shares exercisable at the end of the period	3 years	3 years
Additional disclosures
Total unrecognized compensation expense	$ 10	$ 8
Weighted average remaining vesting period	3 years	3 years