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		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Income Statement Impact [Abstract]
Cost of sales	$ 504,871	$ 522,695	$ 992,822	$ 1,050,207
Operating income	43,440	61,474	110,335	116,076
Income tax expense	12,602	13,401	27,088	25,703
Net earnings attributable to Valmont Industries, Inc.	22,607	39,719	65,536	75,823
Comprehensive income	$ 38,119	$ 36,370	$ 46,899	$ 78,028
Net earnings per diluted share (in dollars per share)	$ 1.06	$ 1.82	$ 3.05	$ 3.46
As Previously Reported
Income Statement Impact [Abstract]
Cost of sales		$ 520,457		$ 1,047,467
Operating income		63,712		118,816
Income tax expense		13,961		26,388
Net earnings attributable to Valmont Industries, Inc.		41,397		77,878
Comprehensive income		$ 38,048		$ 80,083
Net earnings per diluted share (in dollars per share)		$ 1.90		$ 3.56
Adjustment
Income Statement Impact [Abstract]
Cost of sales		$ 2,238		$ 2,740
Operating income		(2,238)		(2,740)
Income tax expense		(560)		(685)
Net earnings attributable to Valmont Industries, Inc.		(1,679)		(2,055)
Comprehensive income		$ (1,679)		$ (2,055)
Net earnings per diluted share (in dollars per share)		$ (0.08)		$ (0.10)