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Segments and Related Information (Schedule of Financial Information by Reportable Segment - Assets) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Assets:      
Assets $ 4,066,576 $ 3,183,132 $ 2,927,300
Reportable Segments [Member] | Communications [Member]      
Assets:      
Assets 1,314,400 1,156,900 1,032,200
Reportable Segments [Member] | Oil and Gas [Member]      
Assets:      
Assets 1,762,600 1,267,200 1,131,400
Reportable Segments [Member] | Electrical Transmission [Member]      
Assets:      
Assets 471,400 419,100 409,100
Reportable Segments [Member] | Power Generation and Industrial [Member]      
Assets:      
Assets 288,600 268,100 252,500
Reportable Segments [Member] | Other [Member]      
Assets:      
Assets 153,200 27,700 34,300
Corporate [Member]      
Assets:      
Assets $ 76,400 $ 44,100 $ 67,800