GUARANTOR/NON-GUARANTOR FINANCIAL INFORMATION - Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 31, 2017	Dec. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 295,622		$ 492,805	$ 493,490	$ 412,516
Receivables, net	500,406		503,677
Inventories	399,905	$ 384,705	420,948
Contract asset - costs and profits in excess of billings	112,620	67,672	16,165
Prepaid expenses and other assets	40,655		27,478
Refundable income taxes	13,182		11,492
Assets held for sale			11,492
Total current assets	1,362,390		1,472,565
Property, plant and equipment, at cost	1,153,843		1,165,687
Less accumulated depreciation and amortization	646,122		646,759
Net property, plant and equipment	507,721		518,928
Goodwill	389,594		337,720
Other intangible assets	178,693		138,599
Investment in subsidiaries and intercompany accounts	0		0
Other assets	124,680		134,438
Total assets	2,563,078		2,602,250
Current liabilities:
Current installments of long-term debt	829		966
Notes payable to banks	3,328		161
Accounts payable	200,468		227,906
Accrued employee compensation and benefits	80,843		84,426
Accrued expenses	106,929	83,072	81,029
Dividends payable	8,310		8,510
Total current liabilities	(400,707)		(402,998)
Deferred income taxes	45,076	38,356	34,906
Long-term debt, excluding current installments	736,185		753,888
Defined benefit pension liability	179,877		189,552
Deferred compensation	48,174		48,526
Other noncurrent liabilities	19,311		20,585
Shareholders’ equity:
Common stock of $1 par value	27,900		27,900
Additional paid-in capital	0		0
Retained earnings	2,022,538	$ 1,964,115	1,954,344
Accumulated other comprehensive loss	(322,554)		(279,022)
Treasury stock	(670,667)		(590,386)
Total Valmont Industries, Inc. shareholders’ equity	1,057,217		1,112,836
Noncontrolling interest in consolidated subsidiaries	76,531		38,959
Total shareholders’ equity	1,133,748		1,151,795	1,152,595	982,586
Total liabilities and shareholders’ equity	2,563,078		2,602,250
Eliminations
Current assets:
Cash and cash equivalents	0		0	0	0
Receivables, net	0		0
Inventories	(2,895)		(3,848)
Contract asset - costs and profits in excess of billings	0		0
Prepaid expenses and other assets	0		0
Refundable income taxes	0
Assets held for sale			0
Total current assets	(2,895)		(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	(3,402,284)		(3,525,162)
Other assets	0		0
Total assets	(3,405,179)		(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
Dividends payable	0		0
Total current liabilities	0		0
Deferred income taxes	0		0
Long-term debt, excluding current installments	(171,128)		(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	(1,127,529)		(1,241,666)
Accumulated other comprehensive loss	270,552		278,085
Treasury stock	0		0
Total Valmont Industries, Inc. shareholders’ equity	(3,234,051)		(3,337,163)
Noncontrolling interest in consolidated subsidiaries	0		0
Total shareholders’ equity	(3,234,051)		(3,337,163)
Total liabilities and shareholders’ equity	(3,405,179)		(3,529,010)
Parent
Current assets:
Cash and cash equivalents	26,292		83,329	118,499	67,225
Receivables, net	154,551		149,221
Inventories	140,195		160,444
Contract asset - costs and profits in excess of billings	49,509		0
Prepaid expenses and other assets	11,586		8,607
Refundable income taxes	13,182
Assets held for sale			11,492
Total current assets	395,315		413,093
Property, plant and equipment, at cost	573,119		557,371
Less accumulated depreciation and amortization	386,282		368,668
Net property, plant and equipment	186,837		188,703
Goodwill	20,108		20,108
Other intangible assets	90		130
Investment in subsidiaries and intercompany accounts	1,345,874		1,416,446
Other assets	49,952		50,773
Total assets	1,998,176		2,089,253
Current liabilities:
Current installments of long-term debt	0		0
Notes payable to banks	0		0
Accounts payable	58,896		69,915
Accrued employee compensation and benefits	42,381		44,086
Accrued expenses	40,948		28,198
Dividends payable	8,310		8,510
Total current liabilities	(150,535)		(150,709)
Deferred income taxes	2,938		20,885
Long-term debt, excluding current installments	733,815		750,821
Defined benefit pension liability	0		0
Deferred compensation	43,460		42,928
Other noncurrent liabilities	10,211		11,074
Shareholders’ equity:
Common stock of $1 par value	27,900		27,900
Additional paid-in capital	0		0
Retained earnings	2,022,538		1,954,344
Accumulated other comprehensive loss	(322,554)		(279,022)
Treasury stock	(670,667)		(590,386)
Total Valmont Industries, Inc. shareholders’ equity	1,057,217		1,112,836
Noncontrolling interest in consolidated subsidiaries	0		0
Total shareholders’ equity	1,057,217		1,112,836
Total liabilities and shareholders’ equity	1,998,176		2,089,253
Guarantors
Current assets:
Cash and cash equivalents	10,628		5,304	4,167	6,071
Receivables, net	72,521		82,995
Inventories	42,840		46,801
Contract asset - costs and profits in excess of billings	39,583		0
Prepaid expenses and other assets	4,433		970
Refundable income taxes	0
Assets held for sale			0
Total current assets	170,005		136,070
Property, plant and equipment, at cost	168,519		160,767
Less accumulated depreciation and amortization	90,810		84,508
Net property, plant and equipment	77,709		76,259
Goodwill	110,562		110,562
Other intangible assets	28,271		30,955
Investment in subsidiaries and intercompany accounts	1,123,215		1,181,537
Other assets	0		0
Total assets	1,509,762		1,535,383
Current liabilities:
Current installments of long-term debt	0		0
Notes payable to banks	0		0
Accounts payable	18,050		18,039
Accrued employee compensation and benefits	7,253		8,749
Accrued expenses	(1,987)		9,621
Dividends payable	0		0
Total current liabilities	(23,316)		(36,409)
Deferred income taxes	0		0
Long-term debt, excluding current installments	171,128		185,078
Defined benefit pension liability	0		0
Deferred compensation	0		0
Other noncurrent liabilities	392		6
Shareholders’ equity:
Common stock of $1 par value	457,950		457,950
Additional paid-in capital	162,906		159,414
Retained earnings	617,466		622,044
Accumulated other comprehensive loss	76,604		74,482
Treasury stock	0		0
Total Valmont Industries, Inc. shareholders’ equity	1,314,926		1,313,890
Noncontrolling interest in consolidated subsidiaries	0		0
Total shareholders’ equity	1,314,926		1,313,890
Total liabilities and shareholders’ equity	1,509,762		1,535,383
Non- Guarantors
Current assets:
Cash and cash equivalents	258,702		404,172	$ 370,824	$ 339,220
Receivables, net	273,334		271,461
Inventories	219,765		217,551
Contract asset - costs and profits in excess of billings	23,528		16,165
Prepaid expenses and other assets	24,636		17,901
Refundable income taxes	0
Assets held for sale			0
Total current assets	799,965		927,250
Property, plant and equipment, at cost	412,205		447,549
Less accumulated depreciation and amortization	169,030		193,583
Net property, plant and equipment	243,175		253,966
Goodwill	258,924		207,050
Other intangible assets	150,332		107,514
Investment in subsidiaries and intercompany accounts	933,195		927,179
Other assets	74,728		83,665
Total assets	2,460,319		2,506,624
Current liabilities:
Current installments of long-term debt	829		966
Notes payable to banks	3,328		161
Accounts payable	123,522		139,952
Accrued employee compensation and benefits	31,209		31,591
Accrued expenses	67,968		43,210
Dividends payable	0		0
Total current liabilities	(226,856)		(215,880)
Deferred income taxes	42,138		14,021
Long-term debt, excluding current installments	2,370		9,836
Defined benefit pension liability	179,877		189,552
Deferred compensation	4,714		5,598
Other noncurrent liabilities	8,708		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	510,063		619,622
Accumulated other comprehensive loss	(347,156)		(352,567)
Treasury stock	0		0
Total Valmont Industries, Inc. shareholders’ equity	1,919,125		2,023,273
Noncontrolling interest in consolidated subsidiaries	76,531		38,959
Total shareholders’ equity	1,995,656		2,062,232
Total liabilities and shareholders’ equity	$ 2,460,319		$ 2,506,624