Stock-Based Compensation - Restricted Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Weighted average grant date fair value
Total stock-based compensation expense (in dollars)	$ 2,996	$ 939	$ 7,121	$ 7,776
Restricted stock
Number of shares
Outstanding at beginning of period (in shares)			753,666
Granted (in shares)			433,459
Vested (in shares)			(120,970)
Forfeited (in shares)			(2,474)
Outstanding at end of period (in shares)	1,063,681		1,063,681
Weighted average grant date fair value
Outstanding at beginning of period (in dollars per share)			$ 12.25
Granted (in dollars per share)			31.56
Vested to common stock (in dollars per share)			12.78
Forfeited (in dollars per share)			12.00
Outstanding at end of period (in dollars per share	$ 20.06		$ 20.06
Total stock-based compensation expense (in dollars)	$ 1,074	$ 68	$ 2,779	$ 5,336
Unrecognized compensation expense (in dollars)	$ 11,535		$ 11,535
Weighted average period expected to be recognized			3 years 2 months 5 days
Restricted stock | Minimum
Stock-Based Compensation
Vesting period			1 year
Restricted stock | Maximum
Stock-Based Compensation
Vesting period			4 years