Property, plant and equipment - Summary of Elements of property, plant and equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 51,039	$ 50,851
Accumulated depreciation	28,255	27,177
Total	22,784	23,674
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	$ 3,045	3,045
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	$ 13,261	13,273
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	$ 25,128	24,953
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	$ 1,574	1,574
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	3,077
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	$ 842	842
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	$ 72	$ 73