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	$ 2,514	$ 1,610
Accrued external research and development expenses	2,055	3,814
Income taxes payable	1	15
Accrued professional fees	867	1,494
Current portion of operating lease liabilities	$ 640
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesCurrent us-gaap:OperatingLeaseLiabilityNoncurrent
Other	$ 67	99
Accrued expenses and other current liabilities	$ 6,144	$ 7,032