Segments and Related Information (Schedule of Financial Information by Reportable Segment - Capital Expenditures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capital Expenditures:
Capital expenditures	$ 123,412	$ 117,114	$ 84,410
Reportable Segments [Member] | Communications [Member]
Capital Expenditures:
Capital expenditures	40,500	28,500	25,800
Reportable Segments [Member] | Oil and Gas [Member]
Capital Expenditures:
Capital expenditures	57,700	64,000	38,100
Reportable Segments [Member] | Electrical Transmission [Member]
Capital Expenditures:
Capital expenditures	14,900	19,800	13,000
Reportable Segments [Member] | Power Generation and Industrial [Member]
Capital Expenditures:
Capital expenditures	5,400	3,400	3,500
Reportable Segments [Member] | Other [Member]
Capital Expenditures:
Capital expenditures	0	300	200
Corporate [Member]
Capital Expenditures:
Capital expenditures	$ 4,900	$ 1,100	$ 3,800