SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Impact of Change from LIFO to FIFO on Statement of Earnings (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 28, 2020	Mar. 30, 2019	Dec. 28, 2019	Dec. 29, 2018
Change in Accounting Estimate [Line Items]
After-tax adjustment to retained earnings			$ 32,854	$ 40,215
Income Statement Impact [Abstract]
Cost of sales	$ 487,951	$ 527,512
Operating income	66,895	54,602
Income tax expense	14,486	12,302
Net earnings attributable to Valmont Industries, Inc.	42,929	36,104
Comprehensive income	$ 8,780	$ 41,658
Net earnings per diluted share (in dollars per share)	$ 1.99	$ 1.64
As Previously Reported
Income Statement Impact [Abstract]
Cost of sales		$ 527,010
Operating income		55,104
Income tax expense		12,427
Net earnings attributable to Valmont Industries, Inc.		36,481
Comprehensive income		$ 42,035
Net earnings per diluted share (in dollars per share)		$ 1.66
Adjustment
Income Statement Impact [Abstract]
Cost of sales		$ 502
Operating income		(502)
Income tax expense		(125)
Net earnings attributable to Valmont Industries, Inc.		(377)
Comprehensive income		$ (377)
Net earnings per diluted share (in dollars per share)		$ (0.02)