EARNINGS PER SHARE - Schedule of Reconciliation of Basic and Diluted Earnings Per Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Basic EPS
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
Shares outstanding basic (in shares)									21,315	21,659	22,306
Per share amount 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
Dilutive Effect of Stock Options
Dilutive Effect of Stock Options									$ 0	$ 0	$ 0
Dilutive effect of stock options number of shares (in shares)									110	110	140
Dilutive effect of stock options (in dollars per share)									$ 0.03	$ 0.03	$ 0.03
Diluted EPS
Diluted EPS									$ 140,693	$ 146,408	$ 101,770
Shares outstanding dilutive (in shares)									21,425	21,769	22,446
Per share amount diluted (in dollars per share)	$ 1.68	$ 1.84	$ 1.06	$ 1.99	$ 1.51	$ 1.75	$ 1.82	$ 1.64	$ 6.57	$ 6.73	$ 4.53