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Stock-Based Compensation - Stock Option Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Number of Shares    
Number of shares outstanding, beginning of period (in shares) 14,573  
Number of shares, granted (in shares) 6,108  
Number of shares, exercised (in shares) (654)  
Number of shares, forfeited/canceled (in shares) (640)  
Number of shares outstanding, end of period (in shares) 19,387 14,573
Number of shares exercisable (in shares) 5,855  
Weighted Average Exercise Price    
Weighted average exercise price, outstanding, beginning of period (in USD per share) $ 4.38  
Weighted average exercise price, granted (in USD per share) 15.17  
Weighted average exercise price, exercised (in USD per share) 2.16  
Weighted average exercise price, forfeited/canceled (in USD per share) 6.81  
Weighted average exercise price, outstanding, end of period (in USD per share) 7.77 $ 4.38
Weighted average exercise price, exercisable (in USD per share) $ 3.55  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]    
Weighted average remaining contractual term (in years), outstanding 8 years 2 months 12 days 8 years 2 months 12 days
Weighted average remaining contractual term (in years), exercisable 6 years 8 months 12 days  
Aggregate intrinsic value, outstanding, beginning of period $ 77,020  
Aggregate intrinsic value, outstanding, end of period 603,127 $ 77,020
Aggregate intrinsic value, exercisable $ 206,850