Property, plant and equipment (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 22,599	$ 21,983
Accumulated depreciation	15,396	14,451
Total	7,203	7,532
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	535	535
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 830	830
Estimated Lives	39 years
Building improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 873	873
Estimated Lives	39 years
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 15,293	13,423
Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	3 years
Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	7 years
Equipment under capital lease [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 226	248
Equipment under capital lease [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	5 years
Equipment under capital lease [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	7 years
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 1,337	1,360
Estimated Lives	5 years
Lab and computer equipment, software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 2,747	2,652
Lab and computer equipment, software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	3 years
Lab and computer equipment, software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	5 years
Furniture & fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 288	289
Furniture & fixtures [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	7 years
Furniture & fixtures [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Lives	8 years
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 119	119
Estimated Lives	3 years
Equipment under construction [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Subtotal	$ 351	$ 1,654