Fair value of financial instruments - Hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Hierarchy for financial instruments measured at fair value on a recurring basis
Short-term investments	$ 4,494
Derivative liability	1,373	$ 2,017
Recurring basis | Fair Value
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	25,757	46,478
Short-term investments	4,494
Total financial assets	30,251	46,478
Earnout liability	37	37
Derivative liability	1,373	2,017
Total financial liabilities	1,410	2,054
Recurring basis | Fair Value | Level 1
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	25,007	27,419
Total financial assets	25,007	27,419
Recurring basis | Fair Value | Level 2
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	750	19,059
Short-term investments	4,494
Total financial assets	5,244	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	1,373	2,017
Total financial liabilities	$ 1,410	$ 2,054