Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 28, 2017
Document Information [Line Items]
Entity Registrant Name	Summit Materials, Inc.
Entity Central Index Key	0001621563
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-30
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Membership interests description	As of October 28, 2017, 100% of Summit Materials, LLC's outstanding limited liability company interests were held by Summit Materials Intermediate Holdings, LLC, its sole member and an indirect subsidiary of Summit Materials, Inc.
Common Class A
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		108,471,857
Common Class B
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		100
Summit Materials, LLC
Document Information [Line Items]
Entity Registrant Name	Summit Materials, LLC
Entity Central Index Key	0001571371
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-30
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Membership interests percentage	100.00%