GUARANTOR/NON-GUARANTOR FINANCIAL INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
GUARANTOR/NON-GUARANTOR FINANCIAL INFORMATION
Parent company's percentage ownership of Guarantors	100.00%								100.00%
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales	$ 683,626	$ 690,340	$ 700,871	$ 692,139	$ 697,363	$ 678,692	$ 682,405	$ 698,684	$ 2,766,976	$ 2,757,144	$ 2,745,967
Cost of sales									2,074,480	2,098,864	2,064,199
Gross profit	170,867	176,086	180,414	165,129	149,701	164,340	174,999	169,240	692,496	658,280	681,768
Selling, general and administrative expenses									454,776	456,000	414,688
Operating income									237,720	202,280	267,080
Other income (expense):
Interest expense									(40,153)	(44,237)	(44,645)
Interest income									3,942	4,668	4,737
Other									8,164	(19,270)	1,292
Total other income (expenses)									(28,047)	(58,839)	(38,616)
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									209,673	143,441	228,464
Income tax expense (benefit):
Current									46,267	44,794	66,390
Deferred income taxes									3,940	(1,659)	39,755
Total income tax expense (benefit)									50,207	43,135	106,145
Earnings before equity in earnings of nonconsolidated subsidiaries									159,466	100,306	122,319
Equity in earnings in nonconsolidated subsidiaries									0	0	0
Net earnings									159,466	100,306	122,319
Less: Earnings attributable to noncontrolling interests									(5,697)	(5,955)	(6,079)
Net earnings attributable to Valmont Industries, Inc.	$ 35,747	$ 40,144	$ 41,397	$ 36,481	$ 17,662	$ 4,448	$ 32,960	$ 39,281	153,769	94,351	116,240
Eliminations
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales									(247,636)	(260,679)	(251,876)
Cost of sales									(247,281)	(262,448)	(252,182)
Gross profit									(355)	1,769	306
Selling, general and administrative expenses									0	0	0
Operating income									(355)	1,769	306
Other income (expense):
Interest expense									11,230	14,815	13,866
Interest income									(11,230)	(14,815)	(13,866)
Other									0	0	0
Total other income (expenses)									0	0	0
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									(355)	1,769	306
Income tax expense (benefit):
Current									2	246	135
Deferred income taxes									0	0	0
Total income tax expense (benefit)									2	246	135
Earnings before equity in earnings of nonconsolidated subsidiaries									(357)	1,523	171
Equity in earnings in nonconsolidated subsidiaries									(112,082)	(105,687)	(106,023)
Net earnings									(112,439)	(104,164)	(105,852)
Less: Earnings attributable to noncontrolling interests									0	0	0
Net earnings attributable to Valmont Industries, Inc.									(112,439)	(104,164)	(105,852)
Parent
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales									1,207,865	1,192,134	1,200,181
Cost of sales									887,403	906,646	898,799
Gross profit									320,462	285,488	301,382
Selling, general and administrative expenses									236,574	192,343	192,182
Operating income									83,888	93,145	109,200
Other income (expense):
Interest expense									(37,984)	(42,524)	(43,642)
Interest income									1,676	791	838
Other									7,805	(17,602)	5,681
Total other income (expenses)									(28,503)	(59,335)	(37,123)
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									55,385	33,810	72,077
Income tax expense (benefit):
Current									8,918	6,310	29,407
Deferred income taxes									(3,120)	1,532	10,307
Total income tax expense (benefit)									5,798	7,842	39,714
Earnings before equity in earnings of nonconsolidated subsidiaries									49,587	25,968	32,363
Equity in earnings in nonconsolidated subsidiaries									104,182	68,383	83,877
Net earnings									153,769	94,351	116,240
Less: Earnings attributable to noncontrolling interests									0	0	0
Net earnings attributable to Valmont Industries, Inc.									153,769	94,351	116,240
Guarantors
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales									563,935	522,366	485,448
Cost of sales									420,287	399,451	375,383
Gross profit									143,648	122,915	110,065
Selling, general and administrative expenses									38,732	51,127	47,955
Operating income									104,916	71,788	62,110
Other income (expense):
Interest expense									(11,150)	(14,815)	(13,866)
Interest income									37	82	42
Other									44	59	58
Total other income (expenses)									(11,069)	(14,674)	(13,766)
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									93,847	57,114	48,344
Income tax expense (benefit):
Current									26,166	14,948	17,928
Deferred income taxes									0	1,791	0
Total income tax expense (benefit)									26,166	16,739	17,928
Earnings before equity in earnings of nonconsolidated subsidiaries									67,681	40,375	30,416
Equity in earnings in nonconsolidated subsidiaries									7,900	37,304	22,146
Net earnings									75,581	77,679	52,562
Less: Earnings attributable to noncontrolling interests									0	0	0
Net earnings attributable to Valmont Industries, Inc.									75,581	77,679	52,562
Non- Guarantors
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales									1,242,812	1,303,323	1,312,214
Cost of sales									1,014,071	1,055,215	1,042,199
Gross profit									228,741	248,108	270,015
Selling, general and administrative expenses									179,470	212,530	174,551
Operating income									49,271	35,578	95,464
Other income (expense):
Interest expense									(2,249)	(1,713)	(1,003)
Interest income									13,459	18,610	17,723
Other									315	(1,727)	(4,447)
Total other income (expenses)									11,525	15,170	12,273
Earnings before income taxes and equity in earnings of nonconsolidated subsidiaries									60,796	50,748	107,737
Income tax expense (benefit):
Current									11,181	23,290	18,920
Deferred income taxes									7,060	(4,982)	29,448
Total income tax expense (benefit)									18,241	18,308	48,368
Earnings before equity in earnings of nonconsolidated subsidiaries									42,555	32,440	59,369
Equity in earnings in nonconsolidated subsidiaries									0	0	0
Net earnings									42,555	32,440	59,369
Less: Earnings attributable to noncontrolling interests									(5,697)	(5,955)	(6,079)
Net earnings attributable to Valmont Industries, Inc.									$ 36,858	$ 26,485	$ 53,290