EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Basic EPS
Net earnings attributable to Valmont Industries, Inc.	$ 35,747	$ 40,144	$ 41,397	$ 36,481	$ 17,662	$ 4,448	$ 32,960	$ 39,281	$ 153,769	$ 94,351	$ 116,240
Shares outstanding basic (in shares)									21,659	22,306	22,520
Per share amount basic (in dollars per share)	$ 1.67	$ 1.86	$ 1.90	$ 1.67	$ 0.80	$ 0.20	$ 1.47	$ 1.74	$ 7.10	$ 4.23	$ 5.16
Dilutive Effect of Stock Options
Dilutive Effect of Stock Options									$ 0	$ 0	$ 0
Dilutive effect of stock options number of shares (in shares)									110	140	218
Dilutive effect of stock options (in dollars per share)									$ 0.04	$ 0.03	$ 0.05
Diluted EPS
Diluted EPS									$ 153,769	$ 94,351	$ 116,240
Shares outstanding dilutive (in shares)									21,769	22,446	22,738
Per share amount diluted (in dollars per share)	$ 1.66	$ 1.85	$ 1.90	$ 1.66	$ 0.80	$ 0.20	$ 1.46	$ 1.72	$ 7.06	$ 4.20	$ 5.11