Stock-Based Compensation - Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Number of shares
Outstanding at beginning of period (in shares)	2,490,114
Granted (in shares)	669,250
Exercised (in shares)	(134,715)
Forfeited (in shares)	(76,370)
Outstanding at end of the period (in shares)	2,948,279
Options vested and expected to vest at end of the period (in shares)	2,948,279
Exercisable at end of period (in shares)	1,540,073
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 15.70
Granted (in dollars per share)	54.90
Exercised (in dollars per share)	12.55
Forfeited (in dollars per share)	47.92
Outstanding at end of period (in dollars per share)	23.91
Options vested and expected to vest at end of period (in dollars per share)	23.91
Exercisable at end of period (in dollars per share)	$ 10.43
Weighted average remaining contractual term
Outstanding	7 years 2 months 12 days
Options vested and expected to vest at of the period	7 years 2 months 12 days
Exercisable	5 years 9 months 18 days
Aggregate intrinsic value
Outstanding (in dollars)	$ 81,869
Options vested and expected to vest at end of period (in dollars)	81,869
Exercisable (in dollars)	60,826
Additional disclosures
Intrinsic value of options exercised (in dollars)	5,959	$ 27,946
Proceeds from stock options exercised (in dollars)	1,536	$ 2,173
Stock options
Additional disclosures
Total unrecognized compensation cost (in dollars)	$ 29,726
Weighted average period expected to be recognized	3 years 2 months 12 days