Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets, net periodic benefit cost	7.00%	7.00%	7.00%
Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets, net periodic benefit cost	7.00%	7.00%	7.00%
Pension Benefits | Minimum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.90%	3.23%	3.61%
Pension Benefits | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.90%	3.23%	3.61%
Pension Benefits | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.02%	3.37%	3.81%
Pension Benefits | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.02%	3.37%	3.81%
Healthcare & Life Ins | Minimum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.87%	3.20%	3.54%
Healthcare & Life Ins | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.87%	3.20%	3.54%
Healthcare & Life Ins | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.91%	3.25%	3.65%
Healthcare & Life Ins | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.91%	3.25%	3.65%