Other current assets and other long-term assets - Summary (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,333	$ 1,261
Deposits and other receivables	2,985	1,841
Interest receivables	792	768
Right-of-use asset	287	0
Total other current assets	5,397	3,870
Deposits paid for research and development	$ 2,600	$ 1,300