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Stock-Based Compensation - Stock Option Activity (Details)
$ / shares in Units, shares in Thousands, $ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Number of Shares    
Number of shares outstanding, beginning of period (in shares) | shares 19,219  
Number of shares, granted (in shares) | shares 0  
Number of shares, exercised (in shares) | shares (3,242)  
Number of shares, forfeited/canceled (in shares) | shares (1,085)  
Number of shares outstanding, end of period (in shares) | shares 14,892 19,219
Number of shares exercisable (in shares) | shares 6,213  
Weighted Average Exercise Price    
Weighted average exercise price, outstanding, beginning of period (in USD per share) | $ / shares $ 7.78  
Weighted average exercise price, granted (in USD per share) | $ / shares 0  
Weighted average exercise price, exercised (in USD per share) | $ / shares 3.93  
Weighted average exercise price, forfeited/canceled (in USD per share) | $ / shares 10.11  
Weighted average exercise price, outstanding, end of period (in USD per share) | $ / shares 8.45 $ 7.78
Weighted average exercise price, exercisable (in USD per share) | $ / shares $ 5.72  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]    
Weighted average remaining contractual term (in years), outstanding 7 years 8 months 12 days 8 years
Weighted average remaining contractual term (in years), exercisable 6 years 10 months 24 days  
Aggregate intrinsic value, outstanding | $ $ 299,201 $ 277,114
Aggregate intrinsic value, exercisable | $ $ 141,797