Other current assets (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 2,310	$ 1,827
Refundable research and development tax credits	0	809
Security deposits and other receivables	482	622
Interest receivables	233	96
Other current assets	3,025	3,354
Deposits paid for research and development	$ 2,000	$ 300