Summary of significant accounting policies, Revenue recognition - Remaining performance obligations (Details) $ in Millions	Dec. 31, 2019 USD ($)
Remaining performance obligations [Abstract]
Remaining performance obligations	$ 5.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Remaining performance obligations [Abstract]
Remaining performance obligations	$ 5.5
Expected timing of satisfaction, period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Remaining performance obligations [Abstract]
Remaining performance obligations	$ 0.2
Expected timing of satisfaction, period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Remaining performance obligations [Abstract]
Remaining performance obligations
Expected timing of satisfaction, period	12 months