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	$ 1,081	$ 2,514
Accrued external research and development expenses	3,142	2,055
Income taxes payable	7	1
Accrued professional fees	717	867
Current portion of operating lease liabilities	636	640
Other	133	67
Accrued expenses and other current liabilities	$ 5,716	$ 6,144