BUSINESS SEGMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Mar. 26, 2016
Business Segments
Net sales	$ 637,473	$ 596,605
Operating income	64,502	62,365
Inventory Valuation [Member]
Business Segments
Operating income	(779)	2,027
Engineered Support Structures Segment
Business Segments
Net sales	160,351	165,959
Operating income	9,213	12,475
Energy and Mining Segment
Business Segments
Net sales	77,972	70,791
Operating income	3,837	1,902
Utility Support Structures Segment
Business Segments
Net sales	174,377	144,344
Operating income	22,708	14,424
Coatings Segment
Business Segments
Net sales	59,332	58,768
Operating income	9,406	11,413
Irrigation Segment
Business Segments
Net sales	165,441	156,743
Operating income	30,291	28,895
Corporate
Business Segments
Operating income	(10,174)	(8,771)
Operating Segment
Business Segments
Sales	673,834	621,035
Operating Segment | Engineered Support Structures Segment
Business Segments
Sales	180,558	176,971
Operating Segment | Engineered Support Structures Segment | Lighting, Traffic, and Roadway Products
Business Segments
Sales	149,082	146,302
Operating Segment | Engineered Support Structures Segment | Communication Products
Business Segments
Sales	31,476	30,669
Operating Segment | Energy and Mining Segment
Business Segments
Sales	77,972	72,449
Operating Segment | Energy and Mining Segment | Offshore Structures
Business Segments
Sales	25,707	22,969
Operating Segment | Energy and Mining Segment | Grinding Media [Member]
Business Segments
Sales	19,594	19,490
Operating Segment | Energy and Mining Segment | Access Systems
Business Segments
Sales	32,671	29,990
Operating Segment | Utility Support Structures Segment
Business Segments
Sales	174,612	144,520
Operating Segment | Utility Support Structures Segment | Steel
Business Segments
Sales	148,408	121,971
Operating Segment | Utility Support Structures Segment | Concrete
Business Segments
Sales	26,204	22,549
Operating Segment | Coatings Segment
Business Segments
Sales	73,468	68,581
Operating Segment | Irrigation Segment
Business Segments
Sales	167,224	158,514
Intersegment
Business Segments
Sales	36,361	24,430
Intersegment | Engineered Support Structures Segment
Business Segments
Sales	20,207	11,012
Intersegment | Energy and Mining Segment
Business Segments
Sales	0	1,658
Intersegment | Utility Support Structures Segment
Business Segments
Sales	235	176
Intersegment | Coatings Segment
Business Segments
Sales	14,136	9,813
Intersegment | Irrigation Segment
Business Segments
Sales	$ 1,783	$ 1,771