SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventories	$ 383,566				$ 420,948				$ 383,566	$ 420,948
Contract asset - costs and profits in excess of billings	112,525				16,165				112,525	16,165
Accrued expenses	91,942				81,029				91,942	81,029
Deferred income taxes	43,489				34,906				43,489	34,906
Retained earnings	2,027,596				1,954,344				2,027,596	1,954,344
Net sales	697,363	$ 678,692	$ 682,405	$ 698,684	714,978	$ 680,779	$ 712,737	$ 637,473	2,757,144	2,745,967	$ 2,521,676
Operating income									202,280	267,080	245,374
Contract with customer, liability, current	4,906				7				4,906	7
Contract with customer, liability, revenue recognized									5,222
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventories	446,267				(36,243)				446,267	(36,243)
Contract asset - costs and profits in excess of billings	24,647				51,507				24,647	51,507
Accrued expenses	86,174				2,043				86,174	2,043
Deferred income taxes	38,492				3,450				38,492	3,450
Retained earnings	2,013,184				9,771				2,013,184	9,771
Net sales									2,720,773
Operating income									196,092
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventories	(62,701)				384,705				(62,701)	384,705
Contract asset - costs and profits in excess of billings	87,878				67,672				87,878	67,672		$ 51,507
Accrued expenses	5,768				83,072				5,768	83,072
Deferred income taxes	4,997				38,356				4,997	38,356
Retained earnings	$ 14,412				1,964,115				14,412	1,964,115
Net sales									36,371
Operating income					13,121				6,188
Offshore and Other Complex Steel Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract asset - costs and profits in excess of billings					$ 16,165					16,165
Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									855,206	856,244	735,617
Operating income									64,766	97,853	71,171
Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									967,358	912,240	891,099
Operating income									34,776	62,960	72,273
Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										256,811	243,877
Operating income									55,325	50,179	46,596
Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									624,761	644,372	567,973
Operating income									97,722	101,498	90,945
Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									23,080	76,300	83,110
Operating income									(913)	2,134	$ 8,730
Transferred at Point in Time
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,861,641
Transferred at Point in Time | Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									16,760
Transferred at Point in Time | Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									922,677
Transferred at Point in Time | Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									286,739
Transferred at Point in Time | Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									612,385
Transferred at Point in Time | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 23,080
Transferred over Time
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										895,503
Transferred over Time | Utility Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										838,446
Transferred over Time | Engineered Support Structures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										44,681
Transferred over Time | Coatings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										0
Transferred over Time | Irrigation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										12,376
Transferred over Time | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales										$ 0