Stock-based Compensation - Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Stock-based compensation expense
Stock-based compensation expense	$ 5,689	$ 7,755	$ 10,535	$ 13,777
Research and development
Stock-based compensation expense
Stock-based compensation expense	2,289	2,366	4,367	4,243
General and administrative
Stock-based compensation expense
Stock-based compensation expense	3,400	5,389	6,168	9,534
Stock options
Stock-based compensation expense
Stock-based compensation expense	5,689	7,632	10,535	13,533
Unvested restricted common stock
Stock-based compensation expense
Stock-based compensation expense	$ 0	$ 123	$ 0	$ 244