Other current assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 3,511	$ 1,879
Security deposits and other receivables	771	759
Interest receivable	157	183
Other current assets	4,439	2,821
Deposits paid for research and development	$ 2,100	$ 3,300