Number Of Businesses Sold

No definition available.

Name:	sum_NumberOfBusinessesSold
Namespace Prefix:	sum_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant.

+ References

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4582445-111684

+ Details

Name:	us-gaap_GainLossOnSaleOfBusiness
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=sum_EastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=sum_WestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sum_SummitMaterialsLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

ACQUISITIONS, DISPOSTIONS, GOODWILL AND INTANGIBLES - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2021 USD ($)	Sep. 26, 2020 USD ($)	Oct. 02, 2021 USD ($) business	Sep. 26, 2020 USD ($)
Business Acquisition [Line Items]
Proceeds from sale of businesses \| $			$ 103,649	$ 0
Gain on sale of business \| $	$ (113)	$ 0	$ 15,319	0
East
Business Acquisition [Line Items]
Number of businesses sold \| business			4
West
Business Acquisition [Line Items]
Number of businesses sold \| business			1
Summit Materials, LLC
Business Acquisition [Line Items]
Proceeds from sale of businesses \| $			$ 103,649	0
Gain on sale of business \| $	$ (113)	$ 0	$ 15,319	$ 0
Summit Materials, LLC \| East
Business Acquisition [Line Items]
Number of businesses sold \| business			4
Summit Materials, LLC \| West
Business Acquisition [Line Items]
Number of businesses sold \| business			1