Segments and Related Information (Schedule of Financial Information by Reportable Segment - Depreciation and Amortization) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Depreciation and Amortization:
Depreciation and amortization	$ 51,676	$ 45,379	$ 101,615	$ 88,282
Reportable Segments [Member] | Communications [Member]
Depreciation and Amortization:
Depreciation and amortization	15,300	13,500	29,900	25,500
Reportable Segments [Member] | Oil and Gas [Member]
Depreciation and Amortization:
Depreciation and amortization	26,100	23,200	51,600	45,100
Reportable Segments [Member] | Electrical Transmission [Member]
Depreciation and Amortization:
Depreciation and amortization	5,100	5,700	10,400	11,400
Reportable Segments [Member] | Power Generation and Industrial [Member]
Depreciation and Amortization:
Depreciation and amortization	3,600	1,400	6,600	2,900
Reportable Segments [Member] | Other [Member]
Depreciation and Amortization:
Depreciation and amortization	0	0	100	100
Corporate [Member]
Depreciation and Amortization:
Depreciation and amortization	$ 1,600	$ 1,600	$ 3,000	$ 3,300