Segments and Related Information (Schedule of Financial Information by Reportable Segment - Depreciation and Amortization) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciation and Amortization:
Depreciation and amortization	$ 188,049	$ 164,915	$ 169,662
Reportable Segments [Member] | Communications [Member]
Depreciation and Amortization:
Depreciation and amortization	53,200	50,300	50,600
Reportable Segments [Member] | Oil and Gas [Member]
Depreciation and Amortization:
Depreciation and amortization	96,700	78,400	84,500
Reportable Segments [Member] | Electrical Transmission [Member]
Depreciation and Amortization:
Depreciation and amortization	22,800	23,200	21,100
Reportable Segments [Member] | Power Generation and Industrial [Member]
Depreciation and Amortization:
Depreciation and amortization	9,100	6,200	6,600
Reportable Segments [Member] | Other [Member]
Depreciation and Amortization:
Depreciation and amortization	100	100	100
Corporate [Member]
Depreciation and Amortization:
Depreciation and amortization	$ 6,100	$ 6,700	$ 6,800