Equity Based Compensation - Equity based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Equity Based Compensation
Total equity-based compensation expense	$ 2,544	$ 499
Research and development expense
Equity Based Compensation
Total equity-based compensation expense	1,792	282
General and administrative expense
Equity Based Compensation
Total equity-based compensation expense	752	217
ESPP
Equity Based Compensation
Total equity-based compensation expense	110
Restricted stock
Equity Based Compensation
Total equity-based compensation expense	562	187
Stock option
Equity Based Compensation
Total equity-based compensation expense	$ 1,872	$ 312