Summary of Organization and Significant Accounting Policies - General Information and Equity Offerings (Details)	12 Months Ended
	Dec. 28, 2019 segment state plant	Dec. 30, 2017	Dec. 29, 2018
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of operating segments	3
Number of reportable segments	3
Number of states in which the entity operates | state	23
Summit Materials, LLC
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of operating segments	3
Number of reportable segments	3
Number of states in which the entity operates | state	23
Summit Materials, LLC | Ohio Valley Asphalt
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Noncontrolling interest elimination (as a percent)	20.00%
Summit Materials, LLC | Continental Cement
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Noncontrolling interest elimination (as a percent)	30.00%
Summit Holdings LP
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Voting power (as a percent)			100.00%
Cement Plant
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of plants | plant	2
Cement Plant | Summit Materials, LLC
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of plants | plant	2
Tax Receivable Agreement | Summit Holdings LP
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Percentage of benefits to be paid on tax receivable agreement	85.00%	85.00%
Minimum | Product
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Collection terms	30 days
Maximum | Product
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Collection terms	60 days
Maximum | Service
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Collection terms	90 days