BUSINESS SEGMENTS (Details)	3 Months Ended								12 Months Ended
	Dec. 29, 2018 USD ($)	Sep. 29, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jul. 01, 2017 USD ($)	Apr. 01, 2017 USD ($)	Dec. 29, 2018 USD ($) segment	Dec. 30, 2017 USD ($)	Dec. 31, 2016 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	$ 697,363,000	$ 678,692,000	$ 682,405,000	$ 698,684,000	$ 714,978,000	$ 680,779,000	$ 712,737,000	$ 637,473,000	$ 2,757,144,000	$ 2,745,967,000	$ 2,521,676,000
Operating income									202,280,000	267,080,000	245,374,000
Interest expense, net									(39,569,000)	(39,908,000)	(41,304,000)
Costs associated with refinancing of debt									(14,820,000)	0	0
Loss from divestiture of grinding media business									(6,084,000)	0	0
Other Nonoperating Income (Expense) excluding Sale of Business and Debt Costs									1,634,000	1,292,000	16,384,000
Other									(19,270,000)	1,292,000	16,384,000
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									143,441,000	228,464,000	220,454,000
Total assets	2,530,274,000				2,602,250,000				2,530,274,000	2,602,250,000	2,391,731,000
Capital expenditures									71,985,000	55,266,000	57,920,000
Depreciation and amortization									82,827,000	84,957,000	82,417,000
Engineered Support Structures
Business Segments
Net sales									967,358,000	912,240,000	891,099,000
Operating income									34,776,000	62,960,000	72,273,000
Total assets	867,735,000				846,881,000				867,735,000	846,881,000	776,161,000
Capital expenditures									26,783,000	16,433,000	13,313,000
Depreciation and amortization									27,274,000	27,637,000	27,824,000
Coatings Segment
Business Segments
Net sales									855,206,000	856,244,000	735,617,000
Operating income									64,766,000	97,853,000	71,171,000
Total assets	700,915,000				597,231,000				700,915,000	597,231,000	544,015,000
Capital expenditures									17,442,000	14,012,000	7,969,000
Depreciation and amortization									23,618,000	25,079,000	24,639,000
Coatings
Business Segments
Net sales										256,811,000	243,877,000
Operating income									55,325,000	50,179,000	46,596,000
Total assets	294,951,000				288,890,000				294,951,000	288,890,000	274,666,000
Capital expenditures									10,320,000	11,080,000	24,873,000
Depreciation and amortization									15,956,000	15,115,000	12,883,000
Irrigation
Business Segments
Net sales									624,761,000	644,372,000	567,973,000
Operating income									97,722,000	101,498,000	90,945,000
Total assets	347,894,000				369,798,000				347,894,000	369,798,000	313,982,000
Capital expenditures									7,249,000	7,055,000	8,836,000
Depreciation and amortization									11,335,000	11,173,000	12,097,000
Other
Business Segments
Net sales									23,080,000	76,300,000	83,110,000
Operating income									(913,000)	2,134,000	8,730,000
Total assets	0				68,934,000				0	68,934,000	65,296,000
Capital expenditures									7,000	2,376,000	1,601,000
Depreciation and amortization									775,000	2,486,000	2,502,000
LIFO Adjustment [Member]
Business Segments
Operating income									(9,892,000)	(5,680,000)	(2,972,000)
Corporate
Business Segments
Operating income									(39,504,000)	(41,864,000)	(41,369,000)
Total assets	318,779,000				430,516,000				318,779,000	430,516,000	417,611,000
Capital expenditures									10,184,000	4,310,000	1,328,000
Depreciation and amortization									3,869,000	3,467,000	2,472,000
Parent
Business Segments
Net sales									1,192,134,000	1,200,181,000	1,126,985,000
Operating income									93,145,000	109,200,000	104,876,000
Loss from divestiture of grinding media business									2,518,000
Other									(17,602,000)	5,681,000	1,480,000
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									33,810,000	72,077,000	62,926,000
Total assets	$ 1,997,073,000				$ 2,089,253,000				1,997,073,000	2,089,253,000
Capital expenditures									25,255,000	20,460,000	9,031,000
Depreciation and amortization									26,155,000	26,237,000	27,096,000
Operating segment
Business Segments
Sales									2,856,150,000	2,844,838,000	2,590,878,000
Operating segment | Engineered Support Structures
Business Segments
Sales									986,880,000	938,102,000	906,719,000
Operating segment | Engineered Support Structures | Lighting, Traffic, and Roadway Products
Business Segments
Sales									706,582,000	633,178,000	612,868,000
Operating segment | Engineered Support Structures | Communication Products
Business Segments
Sales									149,817,000	171,718,000	162,148,000
Operating segment | Engineered Support Structures | Access Systems
Business Segments
Sales									130,481,000	133,206,000	131,703,000
Operating segment | Coatings Segment
Business Segments
Sales									859,173,000	859,115,000	736,364,000
Operating segment | Coatings Segment | Offshore and Other Complex Steel Structures
Business Segments
Sales									92,559,000	100,773,000	107,824,000
Operating segment | Coatings Segment | Steel
Business Segments
Sales									637,979,000	658,604,000	538,284,000
Operating segment | Coatings Segment | Concrete
Business Segments
Sales									111,875,000	99,738,000	90,256,000
Operating segment | Coatings Segment | Engineered Solar Tracking Solutions
Business Segments
Sales									16,760,000	0	0
Operating segment | Coatings
Business Segments
Sales									353,351,000	318,891,000	289,481,000
Operating segment | Irrigation
Business Segments
Sales									633,666,000	652,430,000	575,204,000
Operating segment | Irrigation | North America
Business Segments
Sales									386,683,000	369,832,000	351,436,000
Operating segment | Irrigation | International
Business Segments
Sales									246,983,000	282,598,000	223,768,000
Operating segment | Other
Business Segments
Sales									23,080,000	76,300,000	83,110,000
Intersegment
Business Segments
Sales									99,006,000	98,871,000	69,202,000
Intersegment | Engineered Support Structures
Business Segments
Sales									19,522,000	25,862,000	15,620,000
Intersegment | Coatings Segment
Business Segments
Sales									3,967,000	2,871,000	747,000
Intersegment | Coatings
Business Segments
Sales									66,612,000	62,080,000	45,604,000
Intersegment | Irrigation
Business Segments
Sales									$ 8,905,000	$ 8,058,000	$ 7,231,000