Fair Value - Carrying Value and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 30, 2017	Dec. 31, 2016
Financial Instruments
Current portion of debt	$ 4,765	$ 6,500
Summit Materials, LLC
Financial Instruments
Current portion of debt	4,765	6,500
Level 2
Financial Instruments
Current portion of debt	4,800	6,500
Level 2 \| Fair Value.
Financial Instruments
Long-term debt	1,893,239	1,586,102
Level 2 \| Carrying Value
Financial Instruments
Long-term debt	1,832,455	1,536,065
Level 2 \| Summit Materials, LLC
Financial Instruments
Current portion of debt	4,800	6,500
Level 2 \| Summit Materials, LLC \| Fair Value.
Financial Instruments
Long-term debt	1,893,239	1,586,102
Level 2 \| Summit Materials, LLC \| Carrying Value
Financial Instruments
Long-term debt	1,832,455	1,536,065
Level 3 \| Fair Value.
Financial Instruments
Current portion of deferred consideration and noncompete obligations	13,493	14,874
Long term portion of deferred consideration and noncompete obligations	23,834	30,287
Level 3 \| Carrying Value
Financial Instruments
Current portion of deferred consideration and noncompete obligations	13,493	14,874
Long term portion of deferred consideration and noncompete obligations	23,834	30,287
Level 3 \| Summit Materials, LLC \| Fair Value.
Financial Instruments
Current portion of deferred consideration and noncompete obligations	10,993	12,375
Long term portion of deferred consideration and noncompete obligations	17,938	22,784
Level 3 \| Summit Materials, LLC \| Carrying Value
Financial Instruments
Current portion of deferred consideration and noncompete obligations	10,993	12,375
Long term portion of deferred consideration and noncompete obligations	$ 17,938	$ 22,784

Deferred consideration and noncompete obligations current fair value.

No definition available.

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

- Definition

Deferred consideration and noncompete obligations noncurrent fair value.

+ References

No definition available.

+ Details

Name:	sum_DeferredConsiderationAndNoncompeteObligationsNoncurrentFairValueDisclosure
Namespace Prefix:	sum_
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_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: