EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Sep. 30, 2017	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Basic EPS
Net earnings attributable to Valmont Industries, Inc.	$ 17,662,000	$ 4,448,000	$ 32,960,000	$ 39,281,000	$ (3,611,000)	$ 35,208,000	$ 45,664,000	$ 38,979,000		$ 94,351,000	$ 116,240,000	$ 173,232,000
Shares outstanding basic (in shares)										22,306	22,520	22,562
Per share amount basic (in dollars per share)	$ 0.80	$ 0.20	$ 1.47	$ 1.74	$ (0.16)	$ 1.56	$ 2.03	$ 1.73	$ 5.16	$ 4.23	$ 5.16	$ 7.68
Dilutive Effect of Stock Options
Dilutive Effect of Stock Options										$ 0	$ 0	$ 0
Dilutive effect of stock options number of shares (in shares)										140	218	147
Dilutive effect of stock options (in dollars per share)										$ 0.03	$ 0.05	$ 0.05
Diluted EPS
Diluted EPS										$ 94,351,000	$ 116,240,000	$ 173,232,000
Shares outstanding dilutive (in shares)										22,446	22,738	22,709
Per share amount diluted (in dollars per share)	$ 0.80	$ 0.20	$ 1.46	$ 1.72	$ (0.16)	$ 1.55	$ 2.01	$ 1.72	$ 5.11	$ 4.20	$ 5.11	$ 7.63