Represents the number of reporting units subject to goodwill impairment testing.

No definition available.

Name:	sum_NumberOfReportingUnitsBeingSubjectedToGoodwillImpairmentTest
Namespace Prefix:	sum_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Reporting Unit, Initial Assessment, Fair Value in Excess of Carrying Amount, Number Of Reporting Units

+ References

No definition available.

+ Details

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Namespace Prefix:	sum_
Data Type:	xbrli:integerItemType
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Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

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-Publisher FASB
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-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising 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 2
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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-Section 45
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-Publisher FASB
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 350
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13777-109266

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Details

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Period Type:

Goodwill - Narrative (Details)	3 Months Ended	12 Months Ended
Goodwill - Narrative (Details)	Jan. 02, 2021 reportingUnit	Jan. 02, 2021 USD ($) reportingUnit	Dec. 29, 2018 USD ($)
Goodwill
Number of reporting units with goodwill \| reportingUnit		11
Number of reporting units more likely than not fair value exceeds carrying value \| reportingUnit	7
Goodwill, impairment loss \| $		$ 0
Accumulated impairment losses \| $			$ 68,200,000
Summit Materials, LLC
Goodwill
Number of reporting units with goodwill \| reportingUnit		11
Number of reporting units more likely than not fair value exceeds carrying value \| reportingUnit	7
Goodwill, impairment loss \| $		$ 0
Accumulated impairment losses \| $			$ 68,200,000