SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventories	$ 374,565				$ 383,566				$ 374,565	$ 383,566
Contract assets	141,322				112,525				141,322	112,525
Accrued expenses	201,681				91,942				201,681	91,942
Deferred income tax liabilities, net	47,955				43,489				47,955	43,489
Retained earnings	2,140,948				2,027,596				2,140,948	2,027,596
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
Contract with customer, liability, current	$ 117,945				$ 4,906				117,945	4,906
Contract with customer, liability, revenue recognized									3,921	5,222
Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									885,608	855,206	856,244
Operating income									87,788	64,766	97,853
Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,002,076	967,358	912,240
Operating income									65,627	34,776	62,960
Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									300,640	286,739	256,811
Operating income									51,008	55,325	50,179
Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									578,652	624,761	644,372
Operating income									71,687	97,722	101,498
Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income									0	(913)	$ 2,134
Point in Time
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,865,064	1,861,641
Point in Time | Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									47,450	16,760
Point in Time | Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									952,056	922,677
Point in Time | Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										286,739
Point in Time | Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									564,918	612,385
Point in Time | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									0	23,080
Over Time
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									901,912	895,503
Over Time | Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									838,158	838,446
Over Time | Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									50,020	44,681
Over Time | Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									0	0
Over Time | Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									13,734	12,376
Over Time | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 0	$ 0