Accrued expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued expenses
Research and development costs	$ 6,447	$ 2,384
Employee compensation costs	2,486	2,399
Professional services	1,020	698
Accrued goods and services	309	842
Other	180	89
Patent costs	51	76
Total	$ 10,493	$ 6,488