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Stock-Based Compensation - Stock Options (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 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) 0 0
Forfeited (in shares) 0 0
Outstanding at the end of the period (in shares) 5,000,000 7,000,000
Vested and expected to vest at the end of the period (in shares) 5,000,000 7,000,000
Exercisable at the end of the period (in shares) 3,000,000 6,000,000
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) 11 23
Forfeited (in dollars per share) 0 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 $ 33 $ 3
Vested and expected to vest at the end of the period 98 131
Exercisable at the end of the period $ 88 $ 120
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 $ 11 $ 10
Weighted average remaining vesting period 3 years 3 years