Acquisitions - Acquisitions by Region (Details) - acquistiion	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
West
Business Acquisition [Line Items]
Number of business acquisitions	2	2	5
East
Business Acquisition [Line Items]
Number of business acquisitions	1	0	7
Summit Materials, LLC | West
Business Acquisition [Line Items]
Number of business acquisitions	2	2	5
Summit Materials, LLC | East
Business Acquisition [Line Items]
Number of business acquisitions	1	0	7