COMMITMENTS AND CONTINGENCIES - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
Oct. 02, 2021	Jan. 02, 2021
Loss Contingencies [Line Items]
Anticipated costs	$ 108.0	$ 112.8
Term of purchase commitments	1 year
Site Restoration Obligations \| Other noncurrent liabilities
Loss Contingencies [Line Items]
Site restoration obligation, non-current	$ 32.7	33.6
Site Restoration Obligations \| Accrued expenses.
Loss Contingencies [Line Items]
Site restoration obligation, current	$ 9.6	10.0
Summit Materials, LLC
Loss Contingencies [Line Items]
Term of purchase commitments	1 year
Summit Materials, LLC \| Site Restoration Obligations
Loss Contingencies [Line Items]
Anticipated costs	$ 108.0	112.8
Summit Materials, LLC \| Site Restoration Obligations \| Other noncurrent liabilities
Loss Contingencies [Line Items]
Site restoration obligation, non-current	32.7	33.6
Summit Materials, LLC \| Site Restoration Obligations \| Accrued expenses.
Loss Contingencies [Line Items]
Site restoration obligation, current	$ 9.6	$ 10.0

Total undiscounted anticipated costs for asset retirement obligations.

No definition available.

Name:	sum_AssetRetirementObligationExpectedCost
Namespace Prefix:	sum_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Term of purchase commitment, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	sum_PurchaseCommitmentTerms
Namespace Prefix:	sum_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

Name:	us-gaap_AssetRetirementObligationCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=sum_SiteRestorationObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsPayableAndAccruedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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