GOODWILL AND INTANGIBLE ASSETS - Carrying Values of Trade Names - (Details)	3 Months Ended	12 Months Ended
Sep. 25, 2021 USD ($)	Dec. 26, 2020 USD ($) tradeName
Non-amortized intangible assets
Number of trade names impaired \| tradeName		2
Newmark
Non-amortized intangible assets
Carrying value of trade names	$ 11,111,000	$ 11,111,000
Webforge
Non-amortized intangible assets
Carrying value of trade names	8,035,000	7,972,000
Convert Italia S.p.A
Non-amortized intangible assets
Carrying value of trade names	8,790,000	9,137,000
Valmont SM
Non-amortized intangible assets
Carrying value of trade names	8,386,000	8,720,000
Ingal EPS/Ingal Civil Products
Non-amortized intangible assets
Carrying value of trade names	7,790,000	7,730,000
Walpar
Non-amortized intangible assets
Carrying value of trade names	3,500,000	3,500,000
Shakespeare
Non-amortized intangible assets
Carrying value of trade names	4,000,000	4,000,000
Other
Non-amortized intangible assets
Carrying value of trade names	17,769,000	14,828,000
Trade Name
Non-amortized intangible assets
Carrying value of trade names	69,381,000	66,998,000
Impairment indefinite-lived intangible assets	$ 0	$ 3,900,000

Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

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Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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