Other current assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,551	$ 3,085
Security deposits and other receivables	1,071	1,600
Interest receivable	790	237
Other current assets	3,412	4,922
Deposits paid for research and development	$ 1,100	$ 1,000