GUARANTOR/NON-GUARANTOR FINANCIAL INFORMATION - Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017	Dec. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 722,588	$ 448,222		$ 492,805	$ 412,516
Receivables, net	472,849			503,677
Inventories	381,971		$ 384,705	420,948
Prepaid expenses and other assets	127,843		95,150	43,643
Refundable income taxes	9,841			11,492
Assets held for sale				11,492
Total current assets	1,715,092			1,472,565
Property, plant and equipment, at cost	1,132,074			1,165,687
Less accumulated depreciation and amortization	639,386			646,759
Net property, plant and equipment	492,688			518,928
Goodwill	321,362			337,720
Other intangible assets	124,549			138,599
Investment in subsidiaries and intercompany accounts	0			0
Other assets	125,670			134,438
Total assets	2,779,361			2,602,250
Current liabilities:
Current installments of long-term debt	253,081			966
Notes payable to banks	271			161
Accounts payable	193,612			227,906
Accrued employee compensation and benefits	71,813			84,426
Accrued expenses	79,363		83,072	81,029
Payments of Ordinary Dividends, Common Stock	17,003	16,913
Liabilities held for sale	8,412			8,510
Total current liabilities	(606,552)			(402,998)
Deferred income taxes	34,492		38,356	34,906
Long-term debt, excluding current installments	736,302			753,888
Defined benefit pension liability	183,688			189,552
Deferred compensation	48,118			48,526
Other noncurrent liabilities	20,385			20,585
Shareholders’ equity:
Common stock of $1 par value	27,900			27,900
Additional paid-in capital	0			0
Retained earnings	2,023,919		$ 1,964,115	1,954,344
Accumulated other comprehensive loss	(311,166)			(279,022)
Treasury stock	(628,487)			(590,386)
Total Valmont Industries, Inc. shareholders’ equity	1,112,166			1,112,836
Noncontrolling interest in consolidated subsidiaries	37,658			38,959
Total shareholders’ equity	1,149,824	1,102,446		1,151,795	982,586
Total liabilities and shareholders’ equity	2,779,361			2,602,250
Eliminations
Current assets:
Cash and cash equivalents	0	0		0	0
Receivables, net	0			0
Inventories	(2,927)			(3,848)
Prepaid expenses and other assets	0			0
Refundable income taxes	0
Assets held for sale				0
Total current assets	(2,927)			(3,848)
Property, plant and equipment, at cost	0			0
Less accumulated depreciation and amortization	0			0
Net property, plant and equipment	0			0
Goodwill	0			0
Other intangible assets	0			0
Investment in subsidiaries and intercompany accounts	(3,451,109)			(3,525,162)
Other assets	0			0
Total assets	(3,454,036)			(3,529,010)
Current liabilities:
Current installments of long-term debt	0			0
Notes payable to banks	0			0
Accounts payable	0			0
Accrued employee compensation and benefits	0			0
Accrued expenses	0			0
Payments of Ordinary Dividends, Common Stock	0	0
Liabilities held for sale	0			0
Total current liabilities	0			0
Deferred income taxes	0			0
Long-term debt, excluding current installments	(181,968)			(191,847)
Defined benefit pension liability	0			0
Deferred compensation	0			0
Other noncurrent liabilities	0			0
Shareholders’ equity:
Common stock of $1 par value	(1,106,632)			(1,106,632)
Additional paid-in capital	(1,270,442)			(1,266,950)
Retained earnings	(1,154,848)			(1,241,666)
Accumulated other comprehensive loss	259,854			278,085
Treasury stock	0			0
Total Valmont Industries, Inc. shareholders’ equity	(3,272,068)			(3,337,163)
Noncontrolling interest in consolidated subsidiaries	0			0
Total shareholders’ equity	(3,272,068)			(3,337,163)
Total liabilities and shareholders’ equity	(3,454,036)			(3,529,010)
Parent
Current assets:
Cash and cash equivalents	349,906	94,099		83,329	67,225
Receivables, net	135,850			149,221
Inventories	140,600			160,444
Prepaid expenses and other assets	49,748			8,607
Refundable income taxes	9,841
Assets held for sale				11,492
Total current assets	685,945			413,093
Property, plant and equipment, at cost	566,962			557,371
Less accumulated depreciation and amortization	380,864			368,668
Net property, plant and equipment	186,098			188,703
Goodwill	20,108			20,108
Other intangible assets	103			130
Investment in subsidiaries and intercompany accounts	1,342,466			1,416,446
Other assets	50,346			50,773
Total assets	2,285,066			2,089,253
Current liabilities:
Current installments of long-term debt	252,218			0
Notes payable to banks	0			0
Accounts payable	55,865			69,915
Accrued employee compensation and benefits	37,484			44,086
Accrued expenses	31,803			28,198
Payments of Ordinary Dividends, Common Stock	17,003	16,913
Liabilities held for sale	8,412			8,510
Total current liabilities	(385,782)			(150,709)
Deferred income taxes	(1,652)			20,885
Long-term debt, excluding current installments	733,709			750,821
Defined benefit pension liability	0			0
Deferred compensation	43,413			42,928
Other noncurrent liabilities	11,648			11,074
Shareholders’ equity:
Common stock of $1 par value	27,900			27,900
Additional paid-in capital	0			0
Retained earnings	2,023,919			1,954,344
Accumulated other comprehensive loss	(311,166)			(279,022)
Treasury stock	(628,487)			(590,386)
Total Valmont Industries, Inc. shareholders’ equity	1,112,166			1,112,836
Noncontrolling interest in consolidated subsidiaries	0			0
Total shareholders’ equity	1,112,166			1,112,836
Total liabilities and shareholders’ equity	2,285,066			2,089,253
Guarantors
Current assets:
Cash and cash equivalents	12,698	4,422		5,304	6,071
Receivables, net	86,036			82,995
Inventories	38,888			46,801
Prepaid expenses and other assets	39,066			970
Refundable income taxes	0
Assets held for sale				0
Total current assets	176,688			136,070
Property, plant and equipment, at cost	166,690			160,767
Less accumulated depreciation and amortization	89,134			84,508
Net property, plant and equipment	77,556			76,259
Goodwill	110,562			110,562
Other intangible assets	29,164			30,955
Investment in subsidiaries and intercompany accounts	1,118,506			1,181,537
Other assets	0			0
Total assets	1,512,476			1,535,383
Current liabilities:
Current installments of long-term debt	0			0
Notes payable to banks	0			0
Accounts payable	12,342			18,039
Accrued employee compensation and benefits	5,888			8,749
Accrued expenses	5,963			9,621
Payments of Ordinary Dividends, Common Stock	0	0
Liabilities held for sale	0			0
Total current liabilities	(24,193)			(36,409)
Deferred income taxes	16,883			0
Long-term debt, excluding current installments	175,549			185,078
Defined benefit pension liability	0			0
Deferred compensation	0			0
Other noncurrent liabilities	5			6
Shareholders’ equity:
Common stock of $1 par value	457,950			457,950
Additional paid-in capital	162,906			159,414
Retained earnings	602,675			622,044
Accumulated other comprehensive loss	72,315			74,482
Treasury stock	0			0
Total Valmont Industries, Inc. shareholders’ equity	1,295,846			1,313,890
Noncontrolling interest in consolidated subsidiaries	0			0
Total shareholders’ equity	1,295,846			1,313,890
Total liabilities and shareholders’ equity	1,512,476			1,535,383
Non- Guarantors
Current assets:
Cash and cash equivalents	359,984	349,701		404,172	$ 339,220
Receivables, net	250,963			271,461
Inventories	205,410			217,551
Prepaid expenses and other assets	39,029			34,066
Refundable income taxes	0
Assets held for sale				0
Total current assets	855,386			927,250
Property, plant and equipment, at cost	398,422			447,549
Less accumulated depreciation and amortization	169,388			193,583
Net property, plant and equipment	229,034			253,966
Goodwill	190,692			207,050
Other intangible assets	95,282			107,514
Investment in subsidiaries and intercompany accounts	990,137			927,179
Other assets	75,324			83,665
Total assets	2,435,855			2,506,624
Current liabilities:
Current installments of long-term debt	863			966
Notes payable to banks	271			161
Accounts payable	125,405			139,952
Accrued employee compensation and benefits	28,441			31,591
Accrued expenses	41,597			43,210
Payments of Ordinary Dividends, Common Stock	0	$ 0
Liabilities held for sale	0			0
Total current liabilities	(196,577)			(215,880)
Deferred income taxes	19,261			14,021
Long-term debt, excluding current installments	9,012			9,836
Defined benefit pension liability	183,688			189,552
Deferred compensation	4,705			5,598
Other noncurrent liabilities	8,732			9,505
Shareholders’ equity:
Common stock of $1 par value	648,682			648,682
Additional paid-in capital	1,107,536			1,107,536
Retained earnings	552,173			619,622
Accumulated other comprehensive loss	(332,169)			(352,567)
Treasury stock	0			0
Total Valmont Industries, Inc. shareholders’ equity	1,976,222			2,023,273
Noncontrolling interest in consolidated subsidiaries	37,658			38,959
Total shareholders’ equity	2,013,880			2,062,232
Total liabilities and shareholders’ equity	$ 2,435,855			$ 2,506,624