Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 4,797	$ 3,377
Accrued external research and development expenses	3,265	2,252
Accrued manufacturing facility expenses	4,949
Accrued general and administrative expenses	1,539	1,676
Other	316	311
Accrued expenses and other current liabilities	$ 14,866	$ 7,616