Summary of significant accounting policies, Fixed assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fixed assets [Abstract]
Depreciation expense	$ 1.1	$ 0.9	$ 0.8
Tooling [Member]
Fixed assets [Abstract]
Estimated useful life	5 years
Machinery and Equipment [Member]
Fixed assets [Abstract]
Estimated useful life	10 years
Furniture and Office Equipment [Member] | Minimum [Member]
Fixed assets [Abstract]
Estimated useful life	5 years
Furniture and Office Equipment [Member] | Maximum [Member]
Fixed assets [Abstract]
Estimated useful life	10 years
Computer Software and Equipment [Member] | Minimum [Member]
Fixed assets [Abstract]
Estimated useful life	3 years
Computer Software and Equipment [Member] | Maximum [Member]
Fixed assets [Abstract]
Estimated useful life	7 years