ACQUISITIONS, GOODWILL AND INTANGIBLES - Summary of Goodwill by Reportable Segments (Details) $ in Thousands	9 Months Ended
Sep. 26, 2020 USD ($)
Goodwill [Roll Forward]
Balance—December 28, 2019	$ 1,199,699
Acquisitions	105,298
Foreign currency translation adjustments	(1,911)
Balance—September 26, 2020	1,303,086
West
Goodwill [Roll Forward]
Balance—December 28, 2019	584,617
Acquisitions	105,298
Foreign currency translation adjustments	(1,911)
Balance—September 26, 2020	688,004
East
Goodwill [Roll Forward]
Balance—December 28, 2019	410,426
Acquisitions	0
Foreign currency translation adjustments	0
Balance—September 26, 2020	410,426
Cement
Goodwill [Roll Forward]
Balance—December 28, 2019	204,656
Acquisitions	0
Foreign currency translation adjustments	0
Balance—September 26, 2020	204,656
Summit Materials, LLC
Goodwill [Roll Forward]
Balance—December 28, 2019	1,200,699
Acquisitions	105,298
Foreign currency translation adjustments	(1,911)
Balance—September 26, 2020	1,304,086
Summit Materials, LLC \| West
Goodwill [Roll Forward]
Balance—December 28, 2019	585,617
Acquisitions	105,298
Foreign currency translation adjustments	(1,911)
Balance—September 26, 2020	689,004
Summit Materials, LLC \| East
Goodwill [Roll Forward]
Balance—December 28, 2019	410,426
Acquisitions	0
Foreign currency translation adjustments	0
Balance—September 26, 2020	410,426
Summit Materials, LLC \| Cement
Goodwill [Roll Forward]
Balance—December 28, 2019	204,656
Acquisitions	0
Foreign currency translation adjustments	0
Balance—September 26, 2020	$ 204,656

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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