Fair value of financial instruments - Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Hierarchy for financial instruments measured at fair value on a recurring basis
Short-term investments	$ 1,992
Derivative liability	1,239	$ 2,017
Recurring basis | Fair Value
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	23,284	46,478
Short-term investments	1,992
Total financial assets	25,276	46,478
Earnout liability	37	37
Derivative liability	1,239	2,017
Total financial liabilities	1,276	2,054
Recurring basis | Fair Value | Level 1
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	14,291	27,419
Total financial assets	14,291	27,419
Recurring basis | Fair Value | Level 2
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	8,993	19,059
Short-term investments	1,992
Total financial assets	10,985	19,059
Recurring basis | Fair Value | Level 3
Hierarchy for financial instruments measured at fair value on a recurring basis
Earnout liability	37	37
Derivative liability	1,239	2,017
Total financial liabilities	$ 1,276	$ 2,054