Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 1,161	$ 1,610
Accrued external research and development expenses	2,843	3,814
Income taxes payable	269	15
Accrued professional fees	753	1,494
Current lease liabilities	594
Other	239	99
Accrued expenses and other current liabilities	$ 5,859	$ 7,032