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Goodwill and Other Intangible Assets (Schedule of Goodwill by Segment) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Goodwill [Line Items]    
Goodwill, gross $ 1,258,900  
Accumulated impairment losses (118,400)  
Goodwill, net 1,140,518 $ 1,100,350
Communications [Member]    
Goodwill [Line Items]    
Goodwill, gross 469,900  
Accumulated impairment losses 0  
Goodwill, net 469,900  
Oil and Gas [Member]    
Goodwill [Line Items]    
Goodwill, gross 496,000  
Accumulated impairment losses (118,400)  
Goodwill, net 377,600  
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 143,100  
Accumulated impairment losses 0  
Goodwill, net $ 143,100