Business Segment Information - Net Sales and Operating Income (Loss) by Business Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net sales	$ 68,727	$ 49,796	$ 44,433	$ 34,361	$ 76,106	$ 63,300	$ 66,641	$ 57,170	$ 263,217	$ 197,317	$ 135,052
Net operating income (loss)									21,663	6,972	16,683
Air Pollution Control Segment [Member]
Segment Reporting Information [Line Items]
Net sales									127,707	101,150	88,582
Energy Segment [Member]
Segment Reporting Information [Line Items]
Net sales									70,285	69,355	40,194
Fluid Handling Filtration Segment [Member]
Segment Reporting Information [Line Items]
Net sales									65,638	25,199	6,191
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net sales									(413)	1,613	85
Net operating income (loss)									(14,297)	(17,756)	(6,460)
Operating Segments [Member] | Air Pollution Control Segment [Member]
Segment Reporting Information [Line Items]
Net operating income (loss)									16,803	15,422	14,635
Operating Segments [Member] | Energy Segment [Member]
Segment Reporting Information [Line Items]
Net operating income (loss)									7,799	9,336	7,574
Operating Segments [Member] | Fluid Handling Filtration Segment [Member]
Segment Reporting Information [Line Items]
Net operating income (loss)									13,188	1,443	998
Inter-segment Elimination [Member]
Segment Reporting Information [Line Items]
Net operating income (loss)									$ (1,830)	$ (1,473)	$ (64)