Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Pension Benefits
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 | 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	2.78%	3.90%	3.23%
Pension Benefits | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.78%	3.90%	3.23%
Pension Benefits | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.96%	4.02%	3.37%
Pension Benefits | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.96%	4.02%	3.37%
Healthcare & Life Ins | Minimum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.73%	3.87%	3.20%
Healthcare & Life Ins | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.73%	3.87%	3.20%
Healthcare & Life Ins | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.79%	3.91%	3.25%
Healthcare & Life Ins | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.79%	3.91%	3.25%