Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Benefit Obligations (Detail)	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets, benefit obligation	7.00%	7.00%
Pension Benefits | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets, benefit obligation	7.00%	7.00%
Pension Benefits | Minimum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.84%	2.78%
Pension Benefits | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.84%	2.78%
Pension Benefits | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.14%	2.96%
Pension Benefits | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.14%	2.96%
Healthcare & Life Ins | Minimum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.80%	2.73%
Healthcare & Life Ins | Minimum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.80%	2.73%
Healthcare & Life Ins | Maximum
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.82%	2.79%
Healthcare & Life Ins | Maximum | Summit Materials, LLC
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.82%	2.79%