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	$ 1,530	$ 1,610
Accrued external research and development expenses	2,628	3,814
Income taxes payable	11	15
Accrued professional fees	1,224	1,494
Current lease liabilities	602
Other	40	99
Accrued expenses and other current liabilities	$ 6,035	$ 7,032