Fair value of financial instruments - Hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Hierarchy for financial instruments measured at fair value on a recurring basis
Short-term investments	$ 8,064
Derivative liability (see Note 11)	1,135	$ 2,017
Recurring basis | Fair Value
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	16,830	46,478
Short-term investments	8,064
Total financial assets	24,894	46,478
Earnout liability	30	37
Derivative liability (see Note 11)	1,135	2,017
Total financial liabilities	1,165	2,054
Recurring basis | Fair Value | Level 1
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	12,338	27,419
Short-term investments	1,261
Total financial assets	13,599	27,419
Recurring basis | Fair Value | Level 2
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	4,492	19,059
Short-term investments	6,803
Total financial assets	11,295	19,059
Recurring basis | Fair Value | Level 3
Hierarchy for financial instruments measured at fair value on a recurring basis
Earnout liability	30	37
Derivative liability (see Note 11)	1,135	2,017
Total financial liabilities	$ 1,165	$ 2,054