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		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Income Statement Impact [Abstract]
Cost of sales	$ 543,223	$ 517,053	$ 1,536,045	$ 1,567,260
Operating income	61,479	61,064	171,814	177,140
Income tax expense	12,084	13,063	39,172	38,766
Net earnings attributable to Valmont Industries, Inc.	39,342	38,045	104,878	113,868
Comprehensive income	$ 50,852	$ 25,698	$ 97,751	$ 103,726
Net earnings per diluted share (in dollars per share)	$ 1.84	$ 1.75	$ 4.89	$ 5.22
As Previously Reported
Income Statement Impact [Abstract]
Cost of sales		$ 514,254		$ 1,561,721
Operating income		63,863		182,679
Income tax expense		13,763		40,151
Net earnings attributable to Valmont Industries, Inc.		40,144		118,022
Comprehensive income		$ 27,797		$ 107,880
Net earnings per diluted share (in dollars per share)		$ 1.85		$ 5.41
Adjustment
Income Statement Impact [Abstract]
Cost of sales		$ 2,799		$ 5,539
Operating income		(2,799)		(5,539)
Income tax expense		(700)		(1,385)
Net earnings attributable to Valmont Industries, Inc.		(2,099)		(4,154)
Comprehensive income		$ (2,099)		$ (4,154)
Net earnings per diluted share (in dollars per share)		$ (100)		$ (190)