Stock-Based Compensation - Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Number of shares
Outstanding at beginning of period (in shares)	2,490,114
Granted (in shares)	689,875
Exercised (in shares)	(231,790)
Forfeited (in shares)	(107,350)
Outstanding at end of the period (in shares)	2,840,849
Options vested and expected to vest at end of the period (in shares)	2,840,849
Exercisable at end of period (in shares)	1,585,816
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 15.70
Granted (in dollars per share)	55.05
Exercised (in dollars per share)	11.63
Forfeited (in dollars per share)	49.04
Outstanding at end of period (in dollars per share)	24.33
Options vested and expected to vest at end of period (in dollars per share)	24.33
Exercisable at end of period (in dollars per share)	$ 11.64
Weighted average remaining contractual term
Outstanding	7 years
Options vested and expected to vest at of the period	7 years
Exercisable	5 years 8 months 12 days
Aggregate intrinsic value
Outstanding (in dollars)	$ 88,641
Options vested and expected to vest at end of period (in dollars)	88,641
Exercisable (in dollars)	68,877
Additional disclosures
Intrinsic value of options exercised (in dollars)	10,805	$ 26,905
Proceeds from stock options exercised (in dollars)	2,503	$ 2,590
Stock options
Additional disclosures
Total unrecognized compensation cost (in dollars)	$ 26,556
Weighted average period expected to be recognized	3 years