Accrued expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accrued expenses
Research and development costs	$ 2,774	$ 2,384
Employee compensation costs	1,075	2,399
Professional services	977	698
Accrued goods and services	369	842
Other	312	76
Patent costs	86	89
Total	$ 5,593	$ 6,488