Summary of significant accounting policies - Property and equipment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 3.5	$ 2.2
Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	2 years
Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	5 years
Charging stations and digital media screens | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	5 years
Charging stations and digital media screens | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	10 years
Capitalized research and development equipment
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	2 years
Computers and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	3 years
Computers and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	5 years
Furniture
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	5 years
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	2 years
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	5 years
Development in progress: software
Property, Plant and Equipment [Line Items]
Useful Lives (In Years)	3 years
Construction in progress
Property, Plant and Equipment [Line Items]
Other cost and expense	$ 0.3	$ 0.1