Summary of Significant Accounting Policies - Inventory (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 26, 2020	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Inventory [Line Items]
Retained earnings	$ 2,245,035				$ 2,173,802				$ 2,245,035	$ 2,173,802
Impact of Change from LIFO to FIFO [Abstract]
Cost of sales									2,129,841	2,084,295	$ 2,088,972
Operating income									225,953	227,905	212,172
Income tax expense									49,615	47,753	45,608
Net earnings attributable to Valmont Industries, Inc.	$ 35,815	$ 39,342	$ 22,607	$ 42,929	$ 32,540	$ 38,045	$ 39,719	$ 36,104	140,693	146,408	101,770
Comprehensive (loss) income									$ 147,757	$ 141,676	$ 86,191
Net earnings per dilited share (in dollars per share)	$ 1.68	$ 1.84	$ 1.06	$ 1.99	$ 1.51	$ 1.75	$ 1.82	$ 1.64	$ 6.57	$ 6.73	$ 4.53
Inventory	$ 448,941				$ 418,370				$ 448,941	$ 418,370
Deferred income tax liability	$ 41,689				58,906				$ 41,689	58,906
As Previously Reported
Inventory [Line Items]
Retained earnings					2,140,948					2,140,948
Impact of Change from LIFO to FIFO [Abstract]
Cost of sales										2,074,480	$ 2,098,864
Operating income										237,720	202,280
Income tax expense										50,207	43,135
Net earnings attributable to Valmont Industries, Inc.										153,769	94,351
Comprehensive (loss) income										$ 149,037	$ 78,772
Net earnings per dilited share (in dollars per share)										$ 7.06	$ 4.20
Inventory					374,565					$ 374,565
Deferred income tax liability					47,955					47,955
Adjustment
Inventory [Line Items]
Retained earnings					32,854					32,854
Impact of Change from LIFO to FIFO [Abstract]
Cost of sales										9,815	$ (9,892)
Operating income										(9,815)	9,892
Income tax expense										(2,454)	2,473
Net earnings attributable to Valmont Industries, Inc.										(7,361)	7,419
Comprehensive (loss) income										$ (7,361)	$ 7,419
Net earnings per dilited share (in dollars per share)										$ (0.33)	$ 0.33
Inventory					43,805					$ 43,805
Deferred income tax liability					10,951					10,951
Cumulative Effect, Period of Adoption, Adjustment
Inventory [Line Items]
Retained earnings					$ 32,854					$ 32,854	$ 40,215	$ 32,795