Other current assets and other long-term assets - Summary (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 6,230	$ 1,261
Deposits and other receivables	2,247	1,841
Interest receivable	1,618	768
Right-of-use asset	341	0
Other current assets	10,436	3,870
Deposits paid for research and development	5,200	$ 1,300
Security deposit for lease	$ 300