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Stock-Based Compensation - Stock Options (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options (in shares):      
Outstanding beginning of the period (in shares) 7,532,455    
Granted (in shares) 2,111,253    
Forfeited (in shares) (339,111)    
Exercised (in shares) (1,158,820)    
Outstanding end of the period (in shares) 8,145,777 7,532,455  
Exercisable at end of the period (in shares) 4,607,812    
Weighted-Average Exercise Price (in dollars per share):      
Outstanding beginning of the period, weighted-average exercise price (in dollars per share) $ 12.22    
Granted, weighted-average exercise price (in dollars per share) 16.10    
Forfeited, weighted-average exercise price (in dollars per share) 14.93    
Exercised, weighted-average exercise price (in dollars per share) 10.77    
Outstanding end of the period, weighted-average exercise price (in dollars per share) 13.33 $ 12.22  
Exercisable at the end of the period, weighted-average exercise price (in dollars per share) $ 11.49    
Outstanding, weighted-average remaining contractual term (years) 7 years 7 years 2 months 12 days  
Exercisable, weighted-average remaining contractual term (years) 5 years 8 months 12 days    
Outstanding, aggregate intrinsic value $ 65,913 $ 19,988  
Exercisable, aggregate intrinsic value 45,653    
Options Additional Information:      
Options exercised intrinsic value $ 9,800 $ 3,900 $ 8,400
Options grants in period, weighted average grant date fair value (in dollars per share) $ 6.79 $ 6.79 $ 6.53
Options vested in period fair value $ 10,200 $ 9,300 $ 8,700
Options unrecognized compensation expense $ 19,700    
Employee stock option      
Options Additional Information:      
Unrecognized compensation expense, period for recognition 2 years 6 months    
Class A Common Stock | Employee stock option      
Options Additional Information:      
Expiration period 10 years    
Class A Common Stock | Minimum | Employee stock option      
Options Additional Information:      
Award vesting period 3 years    
Class A Common Stock | Maximum | Employee stock option      
Options Additional Information:      
Award vesting period 4 years