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Stock-Based Compensation - Stock Option Activity (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2019
$ / shares
shares
Dec. 31, 2018
$ / shares
shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Number of options, beginning balance (in shares) | shares 7,235,545  
Number of options, options granted (in shares) | shares 2,781,316  
Number of options, options exercised (in shares) | shares (304,576)  
Number of options, options forfeited (in shares) | shares (220,151)  
Number of options, ending balance (in shares) | shares 9,492,134 7,235,545
Number of options, exercisable (in shares) | shares 4,553,432  
Number of options, vested and expected to vest (in shares) | shares 9,492,134  
Weighted-average exercise price, beginning balance (in dollars per share) | $ / shares $ 7.72  
Weighted-average exercise price, options granted (in dollars per share) | $ / shares 27.50  
Weighted-average exercise price, options exercised (in dollars per share) | $ / shares 2.31  
Weighted-average exercise price, options forfeited (in dollars per share) | $ / shares 9.03  
Weighted-average exercise price, ending balance (in dollars per share) | $ / shares 13.65 $ 7.72
Weighted-average exercise price, exercisable (in dollars per share) | $ / shares 10.03  
Weighted-average exercise price, vested and expected to vest (in dollars per share) | $ / shares $ 13.65  
Weighted-average remaining contract term, outstanding 9 years 26 days 9 years 7 months 13 days
Weighted-average remaining contract term, options granted 9 years 6 months 18 days  
Weighted-average remaining contract term, options exercised 7 years 8 months 19 days  
Weighted-average remaining contract term, options forfeited 8 years 10 months 28 days  
Weighted-average remaining contract term, exercisable 9 years  
Weighted-average remaining contract term, vested and expected to vest 9 years 26 days