GUARANTOR/NON-GUARANTOR FINANCIAL INFORMATION - Condensed Consolidated Statements of Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 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	$ 678,692	$ 680,779	$ 2,059,781	$ 2,030,989
Cost of sales	514,352	517,185	1,551,202	1,519,510
Gross profit	164,340	163,594	508,579	511,479
Selling, general and administrative expenses	125,980	103,504	342,589	308,283
Operating income	38,360	60,090	165,990	203,196
Other income (expense):
Interest expense	(10,954)	(11,190)	(33,819)	(33,312)
Interest income	1,000	1,311	3,713	3,205
Loss from divestiture of grinding media business	0	0	(6,084)	0
Other	(12,324)	350	(17,705)	1,203
Total other income (expenses)	(22,278)	(9,529)	(47,811)	(28,904)
Earnings before income taxes	16,082	50,561	118,179	174,292
Income tax expense (benefit):
Current	10,777	21,163	35,214	50,264
Deferred income taxes	(1,686)	(7,268)	814	79
Total income tax expense (benefit)	9,091	13,895	36,028	50,343
Earnings before equity in earnings of nonconsolidated subsidiaries	6,991	36,666	82,151	123,949
Equity in earnings of nonconsolidated subsidiaries	0	0	0	0
Net earnings	6,991	36,666	82,151	123,949
Less: Earnings attributable to noncontrolling interests	(2,543)	(1,458)	(5,462)	(4,098)
Net earnings attributable to Valmont Industries, Inc.	4,448	35,208	76,689	119,851
Eliminations
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales	(65,566)	(60,820)	(200,754)	(182,956)
Cost of sales	(65,598)	(60,570)	(202,030)	(182,777)
Gross profit	32	(250)	1,276	(179)
Selling, general and administrative expenses	0	0	0	0
Operating income	32	(250)	1,276	(179)
Other income (expense):
Interest expense	3,600	3,989	11,229	10,040
Interest income	(3,600)	(3,989)	(11,229)	(10,040)
Loss from divestiture of grinding media business			0
Other	0	0	0	0
Total other income (expenses)	0	0	0	0
Earnings before income taxes	32	(250)	1,276	(179)
Income tax expense (benefit):
Deferred income taxes			0	0
Total income tax expense (benefit)	(26)	(8)	62	(19)
Earnings before equity in earnings of nonconsolidated subsidiaries	58	(242)	1,214	(160)
Equity in earnings of nonconsolidated subsidiaries	(13,650)	(37,918)	(93,426)	(100,555)
Net earnings	(13,592)	(38,160)	(92,212)	(100,715)
Less: Earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to Valmont Industries, Inc.	(13,592)	(38,160)	(92,212)	(100,715)
Parent
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales	293,070	284,538	906,978	893,988
Cost of sales	227,467	216,039	683,740	666,060
Gross profit	65,603	68,499	223,238	227,928
Selling, general and administrative expenses	51,159	46,451	147,949	143,590
Operating income	14,444	22,048	75,289	84,338
Other income (expense):
Interest expense	(10,511)	(10,884)	(32,788)	(32,672)
Interest income	174	268	655	563
Loss from divestiture of grinding media business			(2,518)
Other	(13,765)	1,379	(15,773)	3,900
Total other income (expenses)	(24,102)	(9,237)	(47,906)	(28,209)
Earnings before income taxes	(9,658)	12,811	27,383	56,129
Income tax expense (benefit):
Deferred income taxes			729	2,065
Total income tax expense (benefit)	(4,497)	5,556	6,181	21,552
Earnings before equity in earnings of nonconsolidated subsidiaries	(5,161)	7,255	21,202	34,577
Equity in earnings of nonconsolidated subsidiaries	9,609	27,953	55,487	85,274
Net earnings	4,448	35,208	76,689	119,851
Less: Earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to Valmont Industries, Inc.	4,448	35,208	76,689	119,851
Guarantors
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales	132,059	113,243	386,793	352,827
Cost of sales	100,130	88,757	293,238	271,620
Gross profit	31,929	24,486	93,555	81,207
Selling, general and administrative expenses	12,908	12,046	37,360	35,555
Operating income	19,021	12,440	56,195	45,652
Other income (expense):
Interest expense	(3,600)	(3,989)	(11,229)	(10,040)
Interest income	47	9	62	33
Loss from divestiture of grinding media business			0
Other	15	11	41	42
Total other income (expenses)	(3,538)	(3,969)	(11,126)	(9,965)
Earnings before income taxes	15,483	8,471	45,069	35,687
Income tax expense (benefit):
Deferred income taxes			1,791	0
Total income tax expense (benefit)	4,732	3,082	12,260	13,184
Earnings before equity in earnings of nonconsolidated subsidiaries	10,751	5,389	32,809	22,503
Equity in earnings of nonconsolidated subsidiaries	4,041	9,965	37,939	15,281
Net earnings	14,792	15,354	70,748	37,784
Less: Earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to Valmont Industries, Inc.	14,792	15,354	70,748	37,784
Non- Guarantors
Condensed consolidated financial information for the Company (Parent), the guarantor subsidiaries and the non-guarantor subsidiaries
Net sales	319,129	343,818	966,764	967,130
Cost of sales	252,353	272,959	776,254	764,607
Gross profit	66,776	70,859	190,510	202,523
Selling, general and administrative expenses	61,913	45,007	157,280	129,138
Operating income	4,863	25,852	33,230	73,385
Other income (expense):
Interest expense	(443)	(306)	(1,031)	(640)
Interest income	4,379	5,023	14,225	12,649
Loss from divestiture of grinding media business			(3,566)
Other	1,426	(1,040)	(1,973)	(2,739)
Total other income (expenses)	5,362	3,677	11,221	9,270
Earnings before income taxes	10,225	29,529	44,451	82,655
Income tax expense (benefit):
Deferred income taxes			(1,706)	(1,986)
Total income tax expense (benefit)	8,882	5,265	17,525	15,626
Earnings before equity in earnings of nonconsolidated subsidiaries	1,343	24,264	26,926	67,029
Equity in earnings of nonconsolidated subsidiaries	0	0	0	0
Net earnings	1,343	24,264	26,926	67,029
Less: Earnings attributable to noncontrolling interests	(2,543)	(1,458)	(5,462)	(4,098)
Net earnings attributable to Valmont Industries, Inc.	$ (1,200)	$ 22,806	$ 21,464	$ 62,931