Stock-Based Compensation - Restricted Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Weighted average grant date fair value
Total stock-based compensation expense (in dollars)	$ 7,225	$ 2,996	$ 20,983	$ 7,121
Restricted stock
Number of shares
Outstanding at beginning of period (in shares)			1,070,061
Granted (in shares)			589,402
Vested (in shares)			(329,375)
Outstanding at end of period (in shares)	1,330,088		1,330,088
Weighted average grant date fair value
Outstanding at beginning of period (in dollars per share)			$ 20.61
Granted (in dollars per share)			54.93
Vested to common stock (in dollars per share)			19.74
Outstanding at end of period (in dollars per share	$ 36.04		$ 36.04
Total stock-based compensation expense (in dollars)	$ 3,550	$ 1,074	$ 9,636	$ 2,779
Unrecognized compensation expense (in dollars)	$ 33,963		$ 33,963
Weighted average period expected to be recognized			3 years