Goodwill and Intangible Assets - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Unit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Aggregate amount of goodwill acquired	$ 242,300,000	$ 242,300,000	$ 241,100,000
Aggregate amount of impairment	75,300,000	75,300,000	70,900,000
Goodwill, impairment charge		$ 4,443,000	53,762,000
Number of reporting units | Unit		3
Goodwill	166,951,000	$ 166,951,000	170,153,000	$ 220,163,000
Intangible assets – indefinite life	19,691,000	19,691,000	22,042,000	26,337,000
Impairment of intangible assets		2,725,000	4,161,000
Asset impairment charges		0
Amortization expense of finite life intangible assets		11,500,000	13,900,000	$ 12,300,000
Amortization expense of finite life intangibles for 2018	10,100,000	10,100,000
Amortization expense of finite life intangibles for 2019	8,900,000	8,900,000
Amortization expense of finite life intangibles for 2020	7,200,000	7,200,000
Amortization expense of finite life intangibles for 2021	5,900,000	5,900,000
Amortization expense of finite life intangibles for 2022	4,900,000	4,900,000
Weighted average amortization period for finite lived assets acquired				8 years 8 months 12 days
Zhongli, Inc. [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Impairment of intangible assets		900,000
Unit Four [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Impairment of intangible assets		500,000
Unit Three [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Impairment of intangible assets		300,000
Energy Segment [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill, impairment charge	4,400,000	4,443,000
Goodwill	72,625,000	72,625,000	75,827,000	$ 72,075,000
Environmental Segment [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill, impairment charge			6,828,000
Goodwill	48,203,000	48,203,000	48,203,000	55,031,000
Four Reporting Units [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill, impairment charge			53,800,000
Intangible assets – indefinite life	$ 13,100,000	$ 13,100,000
Fair value lower than the aggregate carrying value	20.70%	20.70%
Impairment of intangible assets			4,200,000
Three Reporting Units [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill, impairment charge				0
Impairment of intangible assets		$ 2,700,000		$ 3,300,000
PMFG, Inc. [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Decrease in property and equipment			5,500,000
Decrease in current assets			1,700,000
Decrease in deferred income tax liability			1,100,000
Decrease in noncontrolling interest			1,800,000
Goodwill			$ 59,860,000
Unit One [Member] | Environmental Segment [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 32,400,000	$ 32,400,000
Percentage of fair value exceeding carrying value of goodwill		6.00%
Decrease in projected revenue growth rates		0.11%
Decrease in projected operational profit growth rates		0.65%
Unit Two [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Impairment of intangible assets		$ 1,000,000
Unit Two [Member] | Energy Segment [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill	59,900,000	$ 59,900,000
Percentage of fair value exceeding carrying value of goodwill		3.00%
Decrease in projected revenue growth rates		0.04%
Decrease in projected operational profit growth rates		0.16%
Unit Three [Member] | Energy Segment [Member]
Schedule Of Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 5,700,000	$ 5,700,000
Percentage of fair value exceeding carrying value of goodwill		2.00%
Decrease in projected revenue growth rates		0.01%
Decrease in projected operational profit growth rates		0.20%