DEBT - Other (Details) - CAD ($)	Jan. 15, 2015	Mar. 28, 2020	Dec. 28, 2019
Canadian subsidiary credit agreement, Operating activities
Debt Instrument [Line Items]
Revolving credit commitment	$ 6,000,000.0
Canadian subsidiary credit agreement, Capital equipment
Debt Instrument [Line Items]
Revolving credit commitment	$ 500,000
Basis spread on variable rate	0.90%
Canadian subsidiary credit agreement, Guarantees
Debt Instrument [Line Items]
Revolving credit commitment	$ 300,000
Canadian subsidiary credit agreement
Debt Instrument [Line Items]
Amount outstanding		$ 0	$ 0
Prime rate | Canadian subsidiary credit agreement, Operating activities
Debt Instrument [Line Items]
Basis spread on variable rate	0.20%
Summit Materials, LLC | Canadian subsidiary credit agreement, Operating activities
Debt Instrument [Line Items]
Revolving credit commitment	$ 6,000,000.0
Summit Materials, LLC | Canadian subsidiary credit agreement, Capital equipment
Debt Instrument [Line Items]
Revolving credit commitment	$ 500,000
Basis spread on variable rate	0.90%
Summit Materials, LLC | Canadian subsidiary credit agreement, Guarantees
Debt Instrument [Line Items]
Revolving credit commitment	$ 300,000
Summit Materials, LLC | Canadian subsidiary credit agreement
Debt Instrument [Line Items]
Amount outstanding		$ 0	$ 0
Summit Materials, LLC | Prime rate | Canadian subsidiary credit agreement, Operating activities
Debt Instrument [Line Items]
Basis spread on variable rate	0.20%