EARNINGS PER SHARE - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 26, 2020	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Impairment of goodwill and intangible assets (in dollars per share)			$ 0.76
Net earnings attributable to Valmont Industries, Inc.	$ 35,815	$ 39,342	$ 22,607	$ 42,929	$ 32,540	$ 38,045	$ 39,719	$ 36,104	$ 140,693	$ 146,408	$ 101,770
Basic (in dollars per share)	$ 1.69	$ 1.85	$ 1.06	$ 2.00	$ 1.52	$ 1.76	$ 1.83	$ 1.65	$ 6.60	$ 6.76	$ 4.56
Refinancing of long-term debt expense											$ 11,115
Refinancing of long-term debt expense (in dollars per share)											$ 0.50
Outstanding stock options with exercise prices exceeding the market price of common stock, excluded from the computation of diluted earnings per share (in shares)									0	130,704	406,806
Goodwill And Indefinite-lived Intangible Assets
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Impairment of goodwill & intangible assets									$ 16,220		$ 14,736
Impairment of goodwill and intangible assets (in dollars per share)									$ 0.76		$ 0.66
2018 Restructuring Plan
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Valmont Industries, Inc.											$ (37,779)
Basic (in dollars per share)											$ (1.68)
2020 Restructuring Plan
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Valmont Industries, Inc.									$ (17,324)
Basic (in dollars per share)									$ (0.81)
Disposed by sale | Donhad
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Valmont Industries, Inc.											$ 5,350
Basic (in dollars per share)											$ 0.24