Property, plant and equipment (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 50,415	$ 49,714
Accumulated depreciation	22,085	19,253
Total	28,330	30,461
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 | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
Land improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	6 years
Buildings
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,446	1,446
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	39 years
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	25 years
Building improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,045	3,045
Building improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	39 years
Building improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	25 years
Cylinders
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 13,381	13,390
Cylinders | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	30 years
Cylinders | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	15 years
Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 23,866	23,524
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
Equipment under capital lease
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 315	315
Equipment under capital lease | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	7 years
Equipment under capital lease | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
Vehicles
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,580	1,612
Vehicles | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
Vehicles | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
Lab and computer equipment, software
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,090	3,056
Lab and computer equipment, software | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	8 years
Lab and computer equipment, software | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	2 years
Furniture & fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 684	656
Furniture & fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	10 years
Furniture & fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 864	711
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	5 years
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Estimated Lives	3 years
Equipment under construction
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 570	$ 385