Acquisitions - Contractual Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
Jan. 02, 2021	Dec. 28, 2019
Remaining payments on contractual obligations
Noncompete agreements, payment term (in general)	5 years
Minimum
Remaining payments on contractual obligations
Term of payment for deferred consideration		5 years
Maximum
Remaining payments on contractual obligations
Term of payment for deferred consideration		20 years
Noncompete and Deferred Consideration Agreements
Remaining payments on contractual obligations
2021	$ 9,705
2022	3,411
2023	2,657
2024	2,620
2025	2,567
Thereafter	4,454
Total scheduled payments	25,414
Present value adjustments	(4,766)
Total noncompete obligations and deferred consideration	$ 20,648
Summit Materials, LLC
Remaining payments on contractual obligations
Noncompete agreements, payment term (in general)	5 years
Summit Materials, LLC \| Minimum
Remaining payments on contractual obligations
Term of payment for deferred consideration	5 years
Summit Materials, LLC \| Maximum
Remaining payments on contractual obligations
Term of payment for deferred consideration	20 years
Summit Materials, LLC \| Noncompete and Deferred Consideration Agreements
Remaining payments on contractual obligations
2021	$ 7,205
2022	3,411
2023	2,657
2024	2,620
2025	2,567
Thereafter	4,454
Total scheduled payments	22,914
Present value adjustments	(4,704)
Total noncompete obligations and deferred consideration	$ 18,210

Present value of contractual obligations.

No definition available.

Name:	sum_ContractualObligationsPresentValue
Namespace Prefix:	sum_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Term of payment for deferred consideration associated with a business acquisition.

+ References

No definition available.

+ Details

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

- Definition

Noncompete Agreements, Payment Term

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of contractual obligation to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of addition (reduction) to the amount at which a liability could be incurred (settled) in a current transaction between willing parties.

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OtherCommitmentsAxis=sum_NoncompeteAndDeferredConsiderationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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