Stock-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Stock Options
Grant date exercise price (in dollars per share)	$ 42
Stock options
Stock Options
Vesting period	5 years
Expiration period	10 years
Shares
Outstanding at the beginning of the period (in shares)	5,000,000	7,000,000
Granted (in shares)	385,220	0
Exercised (in shares)	1,000,000	1,000,000
Forfeited (in shares)	68,927	0
Outstanding at the end of the period (in shares)	4,000,000	6,000,000
Vested and expected to vest at the end of the period (in shares)	4,000,000	6,000,000
Exercisable at the end of the period (in shares)	3,000,000	4,000,000
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 16	$ 15
Granted (in dollars per share)	42	34
Exercised (in dollars per share)	12	15
Forfeited (in dollars per share)	31	0
Outstanding at the end of the period (in dollars per share)	19	16
Vested and expected to vest at the end of the period (in dollars per share)	19	16
Exercisable at the end of the period (in dollars per share)	$ 15	$ 13
Aggregate intrinsic value
Exercised	$ 36	$ 33
Vested and expected to vest at the end of the period	82	130
Exercisable at the end of the period	$ 74	$ 113
Weighted average remaining option term
Outstanding at the end of the period	5 years	4 years
Vested and expected to vest at the end of the period	5 years	4 years
Exercisable at the end of the period	4 years	3 years
Additional disclosures
Total unrecognized compensation expense	$ 10	$ 9
Weighted average remaining vesting period	3 years	3 years