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ACQUISITIONS - Narrative (Details) - USD ($) $ in Thousands								1 Months Ended			6 Months Ended	12 Months Ended
ACQUISITIONS - Narrative (Details) - USD ($) $ in Thousands	May 13, 2019	Feb. 11, 2019	Dec. 31, 2018	Oct. 18, 2018	Aug. 03, 2018	Aug. 01, 2018	Jan. 26, 2018	Apr. 30, 2019	Jan. 31, 2019	Mar. 30, 2018	Jun. 29, 2019	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017	Mar. 31, 2018
Business Acquisition [Line Items]
Goodwill												$ 428,864	$ 385,207	$ 337,720
Purchase of noncontrolling interest												27,845	$ 5,510	$ 0
Connect-It Wireless, Inc.
Business Acquisition [Line Items]
Cash paid to acquire business	$ 6,034
Goodwill	3,299
United Galvanizing
Business Acquisition [Line Items]
Cash paid to acquire business		$ 26,000
Consideration transferred		28,000
Contingent consideration liability, current		2,000										2,000
Goodwill		12,374
Property, plant, and equipment acquired		10,987
Larson Camouflage
Business Acquisition [Line Items]
Cash paid to acquire business			$ 31,106
Consideration transferred			34,562
Contingent consideration liability, current												3,456
Goodwill			15,346
Property, plant, and equipment acquired			$ 1,151
Contingent consideration liability, as percent of purchase price			10.00%
CSP Coatings Systems Of Auckland
Business Acquisition [Line Items]
Cash paid to acquire business				$ 17,711
Goodwill				5,120
Property, plant, and equipment acquired				7,373
Walpar
Business Acquisition [Line Items]
Cash paid to acquire business					$ 57,805				$ 23,082
Goodwill					$ 45,453
Percentage acquired					72.00%				28.00%
Convert
Business Acquisition [Line Items]
Cash paid to acquire business					$ 43,504						$ 18,700
Goodwill					$ 42,169
Percentage acquired					75.00%
Derit Infrastructure Pvt. Ltd.
Business Acquisition [Line Items]
Cash paid to acquire business						$ 14,700
Torrent Engineering and Equipment
Business Acquisition [Line Items]
Cash paid to acquire business							$ 4,800
Goodwill							3,922
Customer relationships and other intangible assets							$ 4,020
Percentage acquired							60.00%
Annual sales							$ 9,000
Walpar, Convert, Torrent and CSP Coatings
Business Acquisition [Line Items]
Net sales of acquiree												$ 117,296
Valmont SM
Business Acquisition [Line Items]
Percentage acquired								4.80%
Purchase of noncontrolling interest								$ 4,763
Valmont Industria e Commercio Ltda.
Business Acquisition [Line Items]
Percentage acquired															10.00%
Purchase of noncontrolling interest										$ 5,510
Customer Relationships \| Connect-It Wireless, Inc.
Business Acquisition [Line Items]
Customer relationships and other intangible assets	$ 828
Customer Relationships \| United Galvanizing
Business Acquisition [Line Items]
Customer relationships and other intangible assets		$ 3,170
Useful life		10 years
Customer Relationships \| Larson Camouflage
Business Acquisition [Line Items]
Customer relationships and other intangible assets			$ 16,223
Useful life			12 years
Customer Relationships \| CSP Coatings Systems Of Auckland
Business Acquisition [Line Items]
Customer relationships and other intangible assets				$ 3,113
Useful life				10 years
Customer Relationships \| Walpar
Business Acquisition [Line Items]
Useful life					12 years
Reduction in customer relationships									$ 3,500
Patents & Proprietary Technology \| Convert
Business Acquisition [Line Items]
Useful life					15 years
Trade Names \| United Galvanizing
Business Acquisition [Line Items]
Customer relationships and other intangible assets		$ 894