Property, plant and equipment - Summary of Elements of property, plant and equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 52,949	$ 52,284
Accumulated depreciation	32,914	30,374
Total	20,035	21,910
- Land
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,255	1,255
- Land improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 319	319
- Land improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	6 years
- Land improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
- Buildings
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,446	1,446
- Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	25 years
- Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	39 years
- Building improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,099	3,072
- Building improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	25 years
- Building improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	39 years
- Cylinders
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 13,305	13,624
- Cylinders | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	15 years
- Cylinders | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	30 years
- Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 25,221	25,138
- Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
- Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
- Equipment under capital lease
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 315	315
- Equipment under capital lease | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
- Equipment under capital lease | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	7 years
- Vehicles
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,740	1,537
- Vehicles | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
- Vehicles | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
- Lab and computer equipment, software
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,103	3,103
- Lab and computer equipment, software | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	2 years
- Lab and computer equipment, software | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	8 years
- Furniture & fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 679	679
- Furniture & fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
- Furniture & fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
- Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 852	852
- Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
- Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
- Equipment under construction
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,615	$ 944