Fair value of financial instruments - Earnout liability (Details) - Earnout liability $ in Thousands	Dec. 31, 2019 USD ($) Y	Dec. 31, 2018 USD ($) Y
Volatility (annual)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	36	42
Recurring basis | Level 3
Methodologies and significant inputs used in the determination of the fair value
Valuation Method Monte Carlo	us-gaap:ValuationTechniqueOptionPricingModelMember	us-gaap:ValuationTechniqueOptionPricingModelMember
Fair value of liability at valuation date	$ 30	$ 37
Recurring basis | Level 3 | Volatility (annual)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	36	42
Recurring basis | Level 3 | Earnout Target 1 (thousands)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	13,700	13,700
Recurring basis | Level 3 | Earnout Target 2 (thousands)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	18,200	18,200
Recurring basis | Level 3 | Maximum | Risk-free rate (annual)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	2.58	3.29
Recurring basis | Level 3 | Maximum | Time period from valuation until end of earnout (in years)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability | Y	9.5	9.5
Recurring basis | Level 3 | Maximum | Discount rate (in percent)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	23.35	21.68
Recurring basis | Level 3 | Minimum | Risk-free rate (annual)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	1.53	2.53
Recurring basis | Level 3 | Minimum | Time period from valuation until end of earnout (in years)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability | Y	0.5	0.5
Recurring basis | Level 3 | Minimum | Discount rate (in percent)
Methodologies and significant inputs used in the determination of the fair value
Derivative Liability	22.88	21.35