Summary of Organization and Significant Accounting Policies - General Information and Equity Offerings (Details) $ in Millions		1 Months Ended	9 Months Ended
	Jan. 10, 2017 USD ($) acquistiion	Oct. 31, 2017	Sep. 29, 2018 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 Holdings LP
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Voting power (as a percent)			100.00%
Acquisition payments | $	$ 110
Number of business acquisitions | acquistiion	2
Summit Materials, LLC
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Number of operating segments			3
Summit Materials, LLC | Ohio Valley Asphalt
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Minority interest purchased (as a percent)		20.00%