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ACQUISITIONS - Narrative (Details) - USD ($) $ in Thousands						1 Months Ended			12 Months Ended
ACQUISITIONS - Narrative (Details) - USD ($) $ in Thousands	Oct. 18, 2018	Aug. 03, 2018	Aug. 01, 2018	Jan. 26, 2018	Jul. 31, 2017	Mar. 30, 2018	Jun. 30, 2016	Apr. 30, 2016	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016	Jan. 31, 2019	Sep. 29, 2018	Mar. 31, 2018	Jun. 25, 2016
Business Acquisition [Line Items]
Goodwill									$ 385,207	$ 337,720	$ 321,110
Acquisitions, net of cash acquired									143,020	5,362	0
Purchase of noncontrolling interest									(5,510)	$ 0	$ (11,009)
CSP Coatings Systems Of Auckland
Business Acquisition [Line Items]
Cash paid to acquire business	$ 17,711
Property, plant and equipment	7,373
Goodwill	5,120
Walpar, LLC [Member]
Business Acquisition [Line Items]
Cash paid to acquire business		$ 57,805
Goodwill		$ 42,216
Ownership percentage acquired		72.00%
Convert Italia SpA
Business Acquisition [Line Items]
Cash paid to acquire business		$ 43,504
Goodwill		$ 34,280
Ownership percentage acquired		75.00%
Fair value of contingent consideration		$ 11,608
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
Intangible assets				4,020
Goodwill				$ 3,922
Ownership percentage acquired				60.00%
Annual sales				$ 9,000
Aircon Guardrails Private Limited
Business Acquisition [Line Items]
Intangible assets													$ 2,109
Goodwill					$ 3,327
Annual sales					10,000
Acquisitions, net of cash acquired					$ 5,362
Walpar, Convert, Torrent and CSP Coatings
Business Acquisition [Line Items]
Net sales of acquiree									33,973
Net earnings of acquiree									$ 1,566
Valmont Industria e Commercio Ltda.
Business Acquisition [Line Items]
Ownership percentage acquired														10.00%
Purchase of noncontrolling interest						$ (5,510)
IGC Galvanizing Industries (M) Sdn Bhd
Business Acquisition [Line Items]
Ownership percentage acquired								30.00%
Purchase of noncontrolling interest								$ (5,841)
Valmont SM
Business Acquisition [Line Items]
Ownership percentage acquired															5.20%
Purchase of noncontrolling interest							$ (5,168)
Valmont SM \| Maximum
Business Acquisition [Line Items]
Ownership percentage acquired															10.00%
Customer Relationships \| CSP Coatings Systems Of Auckland
Business Acquisition [Line Items]
Intangible assets	$ 3,113
Weighted Average Amortization Period (Years)	10 years
Customer Relationships \| Walpar, LLC [Member]
Business Acquisition [Line Items]
Weighted Average Amortization Period (Years)		14 years
Patents & Proprietary Technology \| Convert Italia SpA
Business Acquisition [Line Items]
Weighted Average Amortization Period (Years)		15 years
Subsequent Event \| Walpar, LLC [Member]
Business Acquisition [Line Items]
Ownership percentage acquired												28.00%