Goodwill - Summary of Goodwill by Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
Jan. 02, 2021	Dec. 28, 2019
Goodwill [Roll Forward]
Beginning balance	$ 1,199,699	$ 1,192,028
Acquisitions	19	5,278
Foreign currency translation adjustments	1,573	2,393
Ending balance	1,201,291	1,199,699
Summit Materials, LLC
Goodwill [Roll Forward]
Beginning balance	1,200,699	1,193,028
Acquisitions	19	5,278
Foreign currency translation adjustments	1,573	2,393
Ending balance	1,202,291	1,200,699
West
Goodwill [Roll Forward]
Beginning balance	584,617	580,567
Acquisitions	19	1,657
Foreign currency translation adjustments	1,573	2,393
Ending balance	586,209	584,617
West \| Summit Materials, LLC
Goodwill [Roll Forward]
Beginning balance	585,617	581,567
Acquisitions	19	1,657
Foreign currency translation adjustments	1,573	2,393
Ending balance	587,209	585,617
East
Goodwill [Roll Forward]
Beginning balance	410,426	406,805
Acquisitions	0	3,621
Foreign currency translation adjustments	0	0
Ending balance	410,426	410,426
East \| Summit Materials, LLC
Goodwill [Roll Forward]
Beginning balance	410,426	406,805
Acquisitions	0	3,621
Foreign currency translation adjustments	0	0
Ending balance	410,426	410,426
Cement
Goodwill [Roll Forward]
Beginning balance	204,656	204,656
Acquisitions	0	0
Foreign currency translation adjustments	0	0
Ending balance	204,656	204,656
Cement \| Summit Materials, LLC
Goodwill [Roll Forward]
Beginning balance	204,656	204,656
Acquisitions	0	0
Foreign currency translation adjustments	0	0
Ending balance	$ 204,656	$ 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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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