EARNINGS PER SHARE (Details 2) - USD ($)	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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Impairment of goodwill & intangible assets										$ 15,780,000	$ 0	$ 0
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
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
Refinancing of long-term debt expense										$ 11,115,000
Refinancing of long-term debt expense (in dollars per share)										$ 0.50
Blended tax rate (percent)	25.00%									21.00%	35.00%	35.00%
Deferred income tax expense	$ 20,372,000										$ 20,372,000
Deferred income tax expense (in usd per share)	$ 0.90
Provision charge for deemed repatriation tax	$ 9,890,000
Provision charge for deemed repatriation tax (in usd per share)	$ 0.44
Deferred expenses related to foreign withholding taxes and US state income taxes	$ 11,673,000									$ (1,881,000)	$ 49,766,000	$ (32,391,000)
Deferred expenses related to foreign withholding taxes and US state income taxes (in usd per share)	$ 0.51
Deferred income tax benefit										$ 30,590,000
Deferred income tax benefit (in dollars per share)										$ 1.35
Provision for valuation allowance										$ 9,888,000
Valuation provision effect (in dollars per share)										$ 0.44
Reversal of contingent liability										$ 16,591,000
Reversal of contingent liability effect (in dollars per share)										$ 0.73
Restructuring expenses										$ 34,031,000
Outstanding stock options with exercise prices exceeding the market price of common stock, excluded from the computation of diluted earnings per share (in shares)									0	406,806		197,303
Goodwill And Indefinite-lived Intangible Assets [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Impairment of goodwill & intangible assets										$ 14,736,000
Impairment of goodwill and intangible assets (in dollars per share										$ 0.66
2018 Restructuring Plan [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Valmont Industries, Inc.										$ (37,779,000)
Basic (in dollars per share)										$ (1.68)
Disposed by sale | Donhad
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Valmont Industries, Inc.										$ 5,350,000
Basic (in dollars per share)										$ 0.24