Debt - Other Narrative (Details)	Jan. 15, 2015 USD ($)	Jan. 02, 2021 USD ($)	Dec. 28, 2019 USD ($)	Jan. 15, 2015 CAD ($)
Canadian subsidiary credit agreement, Operating activities
Debt
Revolving credit commitment				$ 6,000,000.0
Canadian subsidiary credit agreement, Operating activities | Summit Materials, LLC
Debt
Revolving credit commitment	$ 6,000,000.0
Canadian subsidiary credit agreement, Capital equipment
Debt
Revolving credit commitment				500,000
Canadian subsidiary credit agreement, Capital equipment | Summit Materials, LLC
Debt
Revolving credit commitment	500,000
Canadian subsidiary credit agreement, Guarantees
Debt
Revolving credit commitment				$ 400,000
Canadian subsidiary credit agreement, Guarantees | Summit Materials, LLC
Debt
Revolving credit commitment	$ 400,000
Canadian subsidiary credit agreement
Debt
Amount outstanding		$ 0	$ 0
Canadian subsidiary credit agreement | Summit Materials, LLC
Debt
Amount outstanding		$ 0	$ 0
Prime rate | Canadian subsidiary credit agreement, Operating activities
Debt
Basis spread on variable rate	0.20%
Prime rate | Canadian subsidiary credit agreement, Operating activities | Summit Materials, LLC
Debt
Basis spread on variable rate	0.20%
Prime rate | Canadian subsidiary credit agreement, Capital equipment
Debt
Basis spread on variable rate	0.90%
Prime rate | Canadian subsidiary credit agreement, Capital equipment | Summit Materials, LLC
Debt
Basis spread on variable rate	0.90%