BUSINESS SEGMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 30, 2017	Sep. 24, 2016
Business Segments
Net sales	$ 680,779	$ 610,247	$ 2,030,989	$ 1,847,101
Operating income	59,923	53,240	202,715	187,411
Inventory Valuation [Member]
Business Segments
Operating income	(1,626)	(2,066)	(2,839)	(3,192)
Engineered Support Structures Segment
Business Segments
Net sales	209,772	193,108	571,248	554,869
Operating income	16,226	20,323	45,683	53,615
Energy and Mining Segment
Business Segments
Net sales	79,749	81,233	234,928	231,307
Operating income	1,417	3,941	9,195	9,184
Utility Support Structures Segment
Business Segments
Net sales	178,528	150,391	536,545	445,894
Operating income	22,108	16,195	65,005	48,201
Coatings Segment
Business Segments
Net sales	67,680	60,003	191,612	182,183
Operating income	14,577	11,696	36,091	37,132
Irrigation Segment
Business Segments
Net sales	145,050	125,512	496,656	432,848
Operating income	18,235	15,308	83,196	75,216
Corporate
Business Segments
Operating income	(11,014)	(12,157)	(33,616)	(32,745)
Operating Segment
Business Segments
Sales	711,043	633,294	2,132,355	1,917,732
Operating Segment | Engineered Support Structures Segment
Business Segments
Sales	221,508	203,184	619,647	584,071
Operating Segment | Engineered Support Structures Segment | Lighting, Traffic, and Roadway Products
Business Segments
Sales	175,184	159,089	498,034	468,582
Operating Segment | Engineered Support Structures Segment | Communication Products
Business Segments
Sales	46,324	44,095	121,613	115,489
Operating Segment | Energy and Mining Segment
Business Segments
Sales	79,755	81,552	234,934	234,693
Operating Segment | Energy and Mining Segment | Offshore Structures
Business Segments
Sales	25,046	27,330	75,372	76,207
Operating Segment | Energy and Mining Segment | Grinding Media [Member]
Business Segments
Sales	19,800	20,681	60,466	61,189
Operating Segment | Energy and Mining Segment | Access Systems
Business Segments
Sales	34,909	33,541	99,096	97,297
Operating Segment | Utility Support Structures Segment
Business Segments
Sales	179,759	150,667	538,993	446,432
Operating Segment | Utility Support Structures Segment | Steel
Business Segments
Sales	160,948	131,085	471,072	379,157
Operating Segment | Utility Support Structures Segment | Concrete
Business Segments
Sales	18,811	19,582	67,921	67,275
Operating Segment | Coatings Segment
Business Segments
Sales	82,593	70,082	235,842	213,961
Operating Segment | Irrigation Segment
Business Segments
Sales	147,428	127,809	502,939	438,575
Intersegment
Business Segments
Sales	30,264	23,047	101,366	70,631
Intersegment | Engineered Support Structures Segment
Business Segments
Sales	11,736	10,076	48,399	29,202
Intersegment | Energy and Mining Segment
Business Segments
Sales	6	319	6	3,386
Intersegment | Utility Support Structures Segment
Business Segments
Sales	1,231	276	2,448	538
Intersegment | Coatings Segment
Business Segments
Sales	14,913	10,079	44,230	31,778
Intersegment | Irrigation Segment
Business Segments
Sales	$ 2,378	$ 2,297	$ 6,283	$ 5,727