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Stockholders' Equity - Stock Option Activities (Details) - Employee Stock Options - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Outstanding, beginning balance (share) 5,556    
Options granted (shares) 619    
Options exercised (shares) (1,986)    
Options forfeited or expired (shares) (142)    
Outstanding, ending balance (shares) 4,047 5,556  
Weighted Average Exercise Price      
Outstanding, beginning balance, weighted-average exercise price $ 5.23    
Options granted, weighted-average exercise price 18.11    
Options exercised, weighted-average exercise price 3.04    
Options forfeited or expired, weighted-average exercise price 12.83    
Outstanding, ending balance, weighted-average exercise price $ 8.00 $ 5.23  
Weighted average remaining contractual life 6 years 4 months 24 days    
Outstanding, Aggregate intrinsic value $ 68,334    
Vested and expected to vest as of December 31, 2017      
Vested and expected to vest, shares 4,047    
Outstanding, weighted-average exercise price $ 8.00    
Outstanding, weighted average remaining contractual life 6 years 4 months 24 days    
Vested and expected to vest, aggregate intrinsic value $ 68,334    
Additional information pertaining to stock options      
Exercisable, shares 2,911    
Exercisable, weighted-average exercise price $ 6.20    
Exercisable, weighted average remaining contractual life 5 years 7 months 6 days    
Exercisable, aggregate intrinsic value $ 54,380    
Fair market value of common stock $ 24.88    
Weighted average grant date fair value per share of options granted $ 8.81 $ 4.50 $ 2.38
Intrinsic value of options exercised $ 33,820 $ 5,865 $ 3,233
Total fair value of options vested during the period 7,296 3,813 4,824
Cash received from options exercised $ 6,047 $ 4,286 $ 1,268