Equity-Based Compensation - Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity-based compensation
Equity-based compensation expense	$ 2,688	$ 1,486	$ 6,120	$ 3,057
Equity-based compensation expense related to modifications of equity awards	600	300	100	300
Research and development expense
Equity-based compensation
Equity-based compensation expense	577	453	1,794	1,240
General and administrative expense
Equity-based compensation
Equity-based compensation expense	$ 2,111	$ 1,033	$ 4,326	$ 1,817