Segments and Related Information (Schedule of Financial Information by Reportable Segment - Depreciation and Amortization) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Depreciation and Amortization:
Depreciation and amortization	$ 54,863	$ 50,101	$ 156,478	$ 138,384
Reportable Segments [Member] | Communications [Member]
Depreciation and Amortization:
Depreciation and amortization	14,500	13,800	44,400	39,400
Reportable Segments [Member] | Oil and Gas [Member]
Depreciation and Amortization:
Depreciation and amortization	30,600	26,000	82,100	71,100
Reportable Segments [Member] | Electrical Transmission [Member]
Depreciation and Amortization:
Depreciation and amortization	4,800	5,800	15,200	17,300
Reportable Segments [Member] | Power Generation and Industrial [Member]
Depreciation and Amortization:
Depreciation and amortization	3,400	2,900	9,900	5,800
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	$ 4,800	$ 4,700