Goodwill and Other Intangible Assets (Schedule of Goodwill by Segment) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, gross	$ 1,218,300,000
Accumulated impairment losses	(72,900,000)
Goodwill, net	1,145,434,000	$ 1,137,738,000
Communications [Member]
Goodwill [Line Items]
Goodwill, gross	466,200,000.0
Accumulated impairment losses	0
Goodwill, net	466,200,000
Oil and Gas [Member]
Goodwill [Line Items]
Goodwill, gross	460,400,000
Accumulated impairment losses	(72,900,000)
Goodwill, net	387,500,000
Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill, gross	149,900,000
Accumulated impairment losses	0
Goodwill, net	149,900,000
Power Generation and Industrial [Member]
Goodwill [Line Items]
Goodwill, gross	141,800,000
Accumulated impairment losses	0
Goodwill, net	$ 141,800,000