BUSINESS SEGMENTS (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 26, 2020 USD ($)	Sep. 26, 2020 USD ($)	Jun. 27, 2020 USD ($)	Mar. 28, 2020 USD ($)	Dec. 28, 2019 USD ($)	Sep. 28, 2019 USD ($)	Jun. 29, 2019 USD ($)	Mar. 30, 2019 USD ($)	Dec. 26, 2020 USD ($) segment	Dec. 28, 2019 USD ($)	Dec. 29, 2018 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	$ 798,377	$ 733,970	$ 688,808	$ 674,200	$ 683,626	$ 690,340	$ 700,871	$ 692,139	$ 2,895,355	$ 2,766,976	$ 2,757,144
Operating income									225,953	227,905	212,172
Interest expense, net									(38,701)	(36,211)	(39,569)
Costs associated with refinancing of debt									0	0	(14,820)
Loss from divestiture of grinding media business									0	0	(6,084)
Other									5,516	8,164	1,634
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									192,768	199,858	153,333
Total assets	2,953,160				2,807,216				2,953,160	2,807,216	2,583,894
Capital expenditures									106,700	97,425	71,985
Depreciation and amortization									82,892	82,264	82,827
Engineered Support Structures
Business Segments
Net sales									983,523	1,002,076	967,358
Operating income									65,342	65,627	34,776
Total assets	932,565				944,428				932,565	944,428	868,336
Capital expenditures									24,447	25,344	26,783
Depreciation and amortization									25,399	26,280	27,274
Utility Support Structures
Business Segments
Net sales									1,002,138	885,608	855,206
Operating income									100,855	87,788	64,766
Total assets	778,127				742,194				778,127	742,194	700,915
Capital expenditures									34,495	26,306	17,442
Depreciation and amortization									23,641	23,779	23,618
Coatings
Business Segments
Net sales										300,640	286,739
Operating income									42,975	51,008	55,325
Total assets	360,594				363,070				360,594	363,070	294,951
Capital expenditures									22,132	23,610	10,320
Depreciation and amortization									15,793	15,907	15,956
Irrigation
Business Segments
Net sales									640,092	578,652	624,761
Operating income									83,046	71,687	97,722
Total assets	465,322				347,887				465,322	347,887	347,894
Capital expenditures									16,740	15,644	7,249
Depreciation and amortization									12,098	10,943	11,335
Other
Business Segments
Operating income									0	0	(913)
Capital expenditures									0	0	7
Depreciation and amortization									0	0	775
Corporate
Business Segments
Operating income									(66,265)	(48,205)	(39,504)
Total assets	$ 416,552				$ 409,637				416,552	409,637	371,798
Capital expenditures									8,886	6,521	10,184
Depreciation and amortization									5,961	5,355	3,869
Operating segment
Business Segments
Sales									2,989,192	2,855,901	2,856,150
Operating segment | Engineered Support Structures
Business Segments
Sales									995,840	1,012,290	986,880
Operating segment | Engineered Support Structures | Lighting, Traffic, and Highway Safety Products
Business Segments
Sales									717,216	708,853	706,582
Operating segment | Engineered Support Structures | Communication Products
Business Segments
Sales									190,203	188,912	149,817
Operating segment | Engineered Support Structures | Access Systems
Business Segments
Sales									88,421	114,525	130,481
Operating segment | Utility Support Structures
Business Segments
Sales									1,002,209	890,580	859,173
Operating segment | Utility Support Structures | Steel
Business Segments
Sales									635,220	630,892	637,979
Operating segment | Utility Support Structures | Concrete
Business Segments
Sales									160,544	122,032	111,875
Operating segment | Utility Support Structures | Engineered Solar Tracker Solutions
Business Segments
Sales									86,382	47,450	16,760
Operating segment | Utility Support Structures | Offshore and Other Complex Steel Structures
Business Segments
Sales									120,063	90,206	92,559
Operating segment | Coatings
Business Segments
Sales									345,312	367,835	353,351
Operating segment | Irrigation
Business Segments
Sales									645,831	585,196	633,666
Operating segment | Irrigation | North America
Business Segments
Sales									378,424	378,613	386,683
Operating segment | Irrigation | International
Business Segments
Sales									267,407	206,583	246,983
Segment Reconciling Items | Other
Business Segments
Sales									0	0	23,080
Net sales									0	0	23,080
Intersegment
Business Segments
Sales									93,837	88,925	99,006
Intersegment | Engineered Support Structures
Business Segments
Sales									12,317	10,214	19,522
Intersegment | Utility Support Structures
Business Segments
Sales									71	4,972	3,967
Intersegment | Coatings
Business Segments
Sales									75,710	67,195	66,612
Intersegment | Irrigation
Business Segments
Sales									$ 5,739	$ 6,544	$ 8,905