Stock-Based Compensation - Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Number of shares
Outstanding at beginning of period (in shares)	2,883,175
Granted (in shares)	468,250
Exercised (in shares)	(695,216)
Forfeited (in shares)	(89,354)
Outstanding at end of the period (in shares)	2,566,855
Options vested and expected to vest at end of the period (in shares)	2,566,855
Exercisable at end of period (in shares)	1,331,417
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 9.26
Granted (in dollars per share)	35.23
Exercised (in dollars per share)	5.61
Forfeited (in dollars per share)	22.03
Outstanding at end of period (in dollars per share)	14.54
Options vested and expected to vest at end of period (in dollars per share)	14.54
Exercisable at end of period (in dollars per share)	$ 7.80
Weighted average remaining contractual term
Outstanding	7 years 3 months
Options vested and expected to vest at of the period	7 years 3 months
Exercisable	5 years 8 months 12 days
Aggregate intrinsic value
Outstanding (in dollars)	$ 171,084
Options vested and expected to vest at end of period (in dollars)	171,084
Exercisable (in dollars)	97,708
Additional disclosures
Intrinsic value of options exercised (in dollars)	26,905	$ 11,665
Proceeds from stock options exercised (in dollars)	$ 2,590	$ 194
Shares delivered as payment for exercise	32,742	362,440
Stock options exercised in cashless transaction	291,031	956,327
Gross cashless exercise value	$ 1,313	$ 3,187
Stock options
Additional disclosures
Total unrecognized compensation cost (in dollars)	$ 13,674
Weighted average period expected to be recognized	2 years 6 months 18 days