Fair value of financial instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financial assets
Cash equivalents	$ 27,583	$ 32,349
Short-term investments	30,335	48,548
Recurring basis | Total
Financial assets
Cash equivalents	25,627	30,540
Short-term investments	30,335	48,548
Total financial assets	55,962	79,088
Level 1 | Recurring basis
Financial assets
Cash equivalents	25,627	20,540
Total financial assets	25,627	20,540
Level 2 | Recurring basis
Financial assets
Cash equivalents		10,000
Short-term investments	30,335	48,548
Total financial assets	$ 30,335	$ 58,548