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Stockholders' Equity - Summary of Stock Option Activity Under Equity-Based Plans (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Number of Shares    
Balance (in shares) 1,296,863  
Exercised (in shares) (617,157)  
Forfeited (in shares) (15,694)  
Canceled (in shares) (8,497)  
Balance (in shares) 655,515 1,296,863
Exercisable (in shares) 642,631  
Vested and expected to vest (in shares) 655,515  
Weighted Average Exercise Price    
Balance (in dollars per share) $ 12.27  
Exercised (in dollars per share) 11.00  
Forfeited (in dollars per share) 32.81  
Expired (in dollars per share) 32.02  
Balance (in dollars per share) 12.90 $ 12.27
Exercisable (in dollars per share) 12.65  
Vested and expected to vest (in dollars per share) $ 12.90  
Weighted Average Remaining Contractual Term    
Balance (in years) 4 years 10 months 27 days 5 years 10 months 13 days
Exercisable (in years) 4 years 11 months 8 days  
Vested and expected to vest ( in years) 4 years 10 months 27 days  
Aggregate Intrinsic Value    
Balance at $ 19 $ 41
Exercisable 19  
Vested and expected to vest $ 19