Fair value of financial instruments - Hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Hierarchy for financial instruments measured at fair value on a recurring basis
Short-term investments		$ 18,448
Derivative liability	$ 2,017	1,660
Recurring basis | Fair Value
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	46,478	31,969
Short-term investments		18,448
Total financial assets	46,478	50,417
Earnout liability	37	170
Derivative liability	2,017	1,660
Total financial liabilities	2,054	1,830
Recurring basis | Fair Value | Level 1
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	27,419	31,969
Total financial assets	27,419	31,969
Recurring basis | Fair Value | Level 2
Hierarchy for financial instruments measured at fair value on a recurring basis
Cash and cash equivalents	19,059
Short-term investments		18,448
Total financial assets	19,059	18,448
Recurring basis | Fair Value | Level 3
Hierarchy for financial instruments measured at fair value on a recurring basis
Earnout liability	37	170
Derivative liability	2,017	1,660
Total financial liabilities	$ 2,054	$ 1,830