Other current assets and other long-term assets - Summary (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 5,893	$ 1,261
Deposits and other receivables	3,246	1,841
Interest receivable	840	768
Right-of-use asset	204	0
Other current assets	10,183	3,870
Deposits paid for research and development	$ 5,300	$ 1,300