Goodwill and Other Intangible Assets (Rollforward of Goodwill by Segment) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill, gross		$ 1,212,500		$ 1,057,000
Goodwill, accumulated impairment loss	[1]	(74,800)		(68,500)
Goodwill [Roll Forward]
Goodwill, net, beginning balance		995,874	$ 988,500
Additions from new business combinations		138,000
Currency translation adjustments		3,800	1,600
Goodwill, net, ending balance		1,137,738	995,874
Earn-Out Liabilities [Member]
Goodwill [Roll Forward]
Accruals of acquisition-related contingent consideration, net	[2]		5,800
Communications [Member]
Goodwill [Line Items]
Goodwill, gross		466,400		414,900
Goodwill, accumulated impairment loss	[1]	0		0
Goodwill [Roll Forward]
Goodwill, net, beginning balance		420,700	414,900
Additions from new business combinations		45,700
Currency translation adjustments		0	0
Goodwill, net, ending balance		466,400	420,700
Communications [Member] | Earn-Out Liabilities [Member]
Goodwill [Roll Forward]
Accruals of acquisition-related contingent consideration, net	[2]		5,800
Oil and Gas [Member]
Goodwill [Line Items]
Goodwill, gross		460,400		374,600
Goodwill, accumulated impairment loss	[1]	(74,800)		(68,500)
Goodwill [Roll Forward]
Goodwill, net, beginning balance		307,700	306,100
Additions from new business combinations		74,100
Currency translation adjustments		3,800	1,600
Goodwill, net, ending balance		385,600	307,700
Oil and Gas [Member] | Earn-Out Liabilities [Member]
Goodwill [Roll Forward]
Accruals of acquisition-related contingent consideration, net	[2]		0
Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill, gross		149,900		149,900
Goodwill, accumulated impairment loss	[1]	0		0
Goodwill [Roll Forward]
Goodwill, net, beginning balance		149,900	149,900
Additions from new business combinations		0
Currency translation adjustments		0	0
Goodwill, net, ending balance		149,900	149,900
Electrical Transmission [Member] | Earn-Out Liabilities [Member]
Goodwill [Roll Forward]
Accruals of acquisition-related contingent consideration, net	[2]		0
Power Generation and Industrial [Member]
Goodwill [Line Items]
Goodwill, gross		135,800		117,600
Goodwill, accumulated impairment loss	[1]	0		$ 0
Goodwill [Roll Forward]
Goodwill, net, beginning balance		117,600	117,600
Additions from new business combinations		18,200
Currency translation adjustments		0	0
Goodwill, net, ending balance		$ 135,800	117,600
Power Generation and Industrial [Member] | Earn-Out Liabilities [Member]
Goodwill [Roll Forward]
Accruals of acquisition-related contingent consideration, net	[2]		$ 0

[1] Accumulated impairment losses include the effect of currency translation gains and/or losses.

[2] Represents contingent consideration for acquisitions prior to January 1, 2009, which is accrued as incurred, in accordance with U.S. GAAP.