Business Segment Information - Net Sales and Income from Operation by Business Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Net sales	$ 80,985	$ 57,170
Income from operations	2,979	5,492
Air Pollution Control Segment [Member]
Segment Reporting Information [Line Items]
Net sales	41,716	26,748
Energy Segment [Member]
Segment Reporting Information [Line Items]
Net sales	24,349	15,281
Fluid Handling Filtration Segment [Member]
Segment Reporting Information [Line Items]
Net sales	15,194	15,495
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net sales	(274)	(354)
Income from operations	(3,817)	(2,735)
Operating Segments [Member] | Air Pollution Control Segment [Member]
Segment Reporting Information [Line Items]
Income from operations	4,018	3,866
Operating Segments [Member] | Energy Segment [Member]
Segment Reporting Information [Line Items]
Income from operations	420	2,387
Operating Segments [Member] | Fluid Handling Filtration Segment [Member]
Segment Reporting Information [Line Items]
Income from operations	2,956	2,671
Inter-segment Elimination [Member]
Segment Reporting Information [Line Items]
Income from operations	$ (598)	$ (697)