SUMMARY OF ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES - General Information (Details)	9 Months Ended
	Sep. 28, 2019 segment plant
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Number of operating segments	3
Number of reportable segments	3
Cement plant
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Number of plants | plant	2
Summit Materials, LLC
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Number of operating segments	3
Number of reportable segments	3
Summit Materials, LLC | Cement plant
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Number of plants | plant	2
Summit Holdings LP
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Voting power (as a percent)	100.00%