BUSINESS SEGMENTS (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 28, 2019 USD ($)	Sep. 28, 2019 USD ($)	Jun. 29, 2019 USD ($)	Mar. 30, 2019 USD ($)	Dec. 29, 2018 USD ($)	Sep. 29, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 28, 2019 USD ($) segment	Dec. 29, 2018 USD ($)	Dec. 30, 2017 USD ($)
Business Segments
Number of reportable segments | segment									4
Maximum percentage of sales of other businesses and activities to consolidated sales as basis for aggregation									10.00%
Net sales	$ 683,626	$ 690,340	$ 700,871	$ 692,139	$ 697,363	$ 678,692	$ 682,405	$ 698,684	$ 2,766,976	$ 2,757,144	$ 2,745,967
Operating income									237,720	202,280	267,080
Interest expense, net									(36,211)	(39,569)	(39,908)
Costs associated with refinancing of debt									0	(14,820)	0
Loss from divestiture of grinding media business									0	(6,084)	0
Other									8,164	1,634	1,292
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									209,673	143,441	228,464
Total assets	2,763,411				2,530,274				2,763,411	2,530,274	2,602,250
Capital expenditures									97,425	71,985	55,266
Depreciation and amortization									82,264	82,827	84,957
Engineered Support Structures
Business Segments
Net sales									1,002,076	967,358	912,240
Operating income									65,627	34,776	62,960
Total assets	943,841				867,735				943,841	867,735	846,881
Capital expenditures									25,344	26,783	16,433
Depreciation and amortization									26,280	27,274	27,637
Utility Support Structures
Business Segments
Net sales									885,608	855,206	856,244
Operating income									87,788	64,766	97,853
Total assets	742,194				700,915				742,194	700,915	597,231
Capital expenditures									26,306	17,442	14,012
Depreciation and amortization									23,779	23,618	25,079
Coatings
Business Segments
Net sales									300,640	286,739	256,811
Operating income									51,008	55,325	50,179
Total assets	363,070				294,951				363,070	294,951	288,890
Capital expenditures									23,610	10,320	11,080
Depreciation and amortization									15,907	15,956	15,115
Irrigation
Business Segments
Net sales									578,652	624,761	644,372
Operating income									71,687	97,722	101,498
Total assets	347,887				347,894				347,887	347,894	369,798
Capital expenditures									15,644	7,249	7,055
Depreciation and amortization									10,943	11,335	11,173
Other
Business Segments
Operating income									0	(913)	2,134
Total assets	0				0				0	0	68,934
Capital expenditures									0	7	2,376
Depreciation and amortization									0	775	2,486
Segment Reconciling Items
Business Segments
Net sales											76,300
Operating income											(41,864)
Total assets	$ 366,419				$ 318,779				366,419	318,779	430,516
Capital expenditures									6,521	10,184	4,310
Depreciation and amortization									5,355	3,869	3,467
Operating segment
Business Segments
Sales									2,855,901	2,856,150	2,844,838
Operating segment | Engineered Support Structures
Business Segments
Sales									1,012,290	986,880	938,102
Operating segment | Engineered Support Structures | Lighting, Traffic, and Highway Safety Products
Business Segments
Sales									708,853	706,582	633,178
Operating segment | Engineered Support Structures | Communication Products
Business Segments
Sales									188,912	149,817	171,718
Operating segment | Engineered Support Structures | Access Systems
Business Segments
Sales									114,525	130,481	133,206
Operating segment | Utility Support Structures
Business Segments
Sales									890,580	859,173	859,115
Operating segment | Utility Support Structures | Steel
Business Segments
Sales									630,892	637,979	658,604
Operating segment | Utility Support Structures | Concrete
Business Segments
Sales									122,032	111,875	99,738
Operating segment | Utility Support Structures | Engineered Solar Tracker Solutions
Business Segments
Sales									47,450	16,760	0
Operating segment | Utility Support Structures | Offshore and Other Complex Steel Structures
Business Segments
Sales									90,206	92,559	100,773
Operating segment | Coatings
Business Segments
Sales									367,835	353,351	318,891
Operating segment | Irrigation
Business Segments
Sales									585,196	633,666	652,430
Operating segment | Irrigation | North America
Business Segments
Sales									378,613	386,683	369,832
Operating segment | Irrigation | International
Business Segments
Sales									206,583	246,983	282,598
Segment Reconciling Items
Business Segments
Operating income									(48,205)	(39,504)
Segment Reconciling Items | Other
Business Segments
Sales									0	23,080	76,300
Net sales									0	23,080
Intersegment
Business Segments
Sales									88,925	99,006	98,871
Intersegment | Engineered Support Structures
Business Segments
Sales									10,214	19,522	25,862
Intersegment | Utility Support Structures
Business Segments
Sales									4,972	3,967	2,871
Intersegment | Coatings
Business Segments
Sales									67,195	66,612	62,080
Intersegment | Irrigation
Business Segments
Sales									6,544	8,905	8,058
Inventory Valuation and Obsolescence | LIFO Adjustment
Business Segments
Operating income									$ 9,815	$ (9,892)	$ (5,680)