Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 2,717	$ 5,045
Accrued external research and development expenses	5,087	6,715
Accrued manufacturing facility expenses	1,277	2,824
Accrued general and administrative expenses	916	1,136
Other	294	322
Accrued expenses and other current liabilities	$ 10,291	$ 16,042