Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 5,045	$ 3,377
Accrued external research and development expenses	6,715	2,252
Accrued manufacturing facility expenses	2,824
Accrued general and administrative expenses	1,136	1,676
Accrued lease liability, current portion		4,502
Other	322	311
Accrued expenses and other current liabilities	$ 16,042	$ 12,118