BUSINESS SEGMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2017	Jun. 25, 2016	Jul. 01, 2017	Jun. 25, 2016
Business Segments
Net sales	$ 712,737	$ 640,249	$ 1,350,210	$ 1,236,854
Operating income	78,290	71,806	142,792	134,171
Inventory Valuation [Member]
Business Segments
Operating income	(434)	(3,153)	(1,213)	(1,126)
Engineered Support Structures Segment
Business Segments
Net sales	201,125	195,802	361,476	361,761
Operating income	20,244	20,817	29,457	33,292
Energy and Mining Segment
Business Segments
Net sales	77,207	79,283	155,179	150,074
Operating income	3,941	3,341	7,778	5,243
Utility Support Structures Segment
Business Segments
Net sales	183,640	151,159	358,017	295,503
Operating income	20,189	17,582	42,897	32,006
Coatings Segment
Business Segments
Net sales	64,600	63,412	123,932	122,180
Operating income	12,108	14,023	21,514	25,436
Irrigation Segment
Business Segments
Net sales	186,165	150,593	351,606	307,336
Operating income	34,670	31,013	64,961	59,908
Corporate
Business Segments
Operating income	(12,428)	(11,817)	(22,602)	(20,588)
Operating Segment
Business Segments
Sales	747,478	663,403	1,421,312	1,284,438
Operating Segment | Engineered Support Structures Segment
Business Segments
Sales	217,581	203,916	398,139	380,887
Operating Segment | Engineered Support Structures Segment | Lighting, Traffic, and Roadway Products
Business Segments
Sales	173,768	163,191	322,850	309,493
Operating Segment | Engineered Support Structures Segment | Communication Products
Business Segments
Sales	43,813	40,725	75,289	71,394
Operating Segment | Energy and Mining Segment
Business Segments
Sales	77,207	80,692	155,179	153,141
Operating Segment | Energy and Mining Segment | Offshore Structures
Business Segments
Sales	24,619	25,908	50,326	48,877
Operating Segment | Energy and Mining Segment | Grinding Media [Member]
Business Segments
Sales	21,072	21,018	40,666	40,508
Operating Segment | Energy and Mining Segment | Access Systems
Business Segments
Sales	31,516	33,766	64,187	63,756
Operating Segment | Utility Support Structures Segment
Business Segments
Sales	184,622	151,245	359,234	295,765
Operating Segment | Utility Support Structures Segment | Steel
Business Segments
Sales	161,716	126,101	310,124	248,072
Operating Segment | Utility Support Structures Segment | Concrete
Business Segments
Sales	22,906	25,144	49,110	47,693
Operating Segment | Coatings Segment
Business Segments
Sales	79,781	75,298	153,249	143,879
Operating Segment | Irrigation Segment
Business Segments
Sales	188,287	152,252	355,511	310,766
Intersegment
Business Segments
Sales	34,741	23,154	71,102	47,584
Intersegment | Engineered Support Structures Segment
Business Segments
Sales	16,456	8,114	36,663	19,126
Intersegment | Energy and Mining Segment
Business Segments
Sales	0	1,409	0	3,067
Intersegment | Utility Support Structures Segment
Business Segments
Sales	982	86	1,217	262
Intersegment | Coatings Segment
Business Segments
Sales	15,181	11,886	29,317	21,699
Intersegment | Irrigation Segment
Business Segments
Sales	$ 2,122	$ 1,659	$ 3,905	$ 3,430