Goodwill and Other Intangible Assets (Schedule of Goodwill by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, gross	$ 1,222,600
Accumulated impairment losses	(72,800)
Goodwill, net	1,149,795	$ 1,137,738
Communications [Member]
Goodwill [Line Items]
Goodwill, gross	467,800
Accumulated impairment losses	0
Goodwill, net	467,800
Oil and Gas [Member]
Goodwill [Line Items]
Goodwill, gross	462,500
Accumulated impairment losses	(72,800)
Goodwill, net	389,700
Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill, gross	149,900
Accumulated impairment losses	0
Goodwill, net	149,900
Power Generation and Industrial [Member]
Goodwill [Line Items]
Goodwill, gross	142,400
Accumulated impairment losses	0
Goodwill, net	$ 142,400