Property, plant and equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 49,714	$ 21,983
Accumulated depreciation	19,253	14,451
Total	30,461	7,532
Land
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,255	535
Buildings
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,446	830
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	39 years
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	25 years
Building improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,045	873
Building improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	39 years
Building improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	25 years
Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 23,524	13,423
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Equipment under capital lease
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 315	248
Equipment under capital lease | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	7 years
Equipment under capital lease | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Vehicles
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,612	1,360
Vehicles | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Vehicles | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Lab and computer equipment, software
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,056	2,652
Lab and computer equipment, software | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	8 years
Lab and computer equipment, software | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	2 years
Furniture & fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 656	289
Furniture & fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Furniture & fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 711	119
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Equipment under construction
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 385	1,654
Cylinders
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 13,390	0
Cylinders | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	30 years
Cylinders | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Land improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 319	$ 0
Land improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Land improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	6 years