Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 71	$ 60	$ 67
Interest cost	733	928	898
Amortization of gain	326	202	312
Expected return on plan assets	(1,221)	(1,244)	(1,284)
Amortization of prior service credit	0	0	0
Net periodic (expense) benefit cost	(91)	(54)	(7)
Pension Benefits | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	71	60	67
Interest cost	733	928	898
Amortization of gain	326	202	312
Expected return on plan assets	(1,221)	(1,244)	(1,284)
Amortization of prior service credit	0	0	0
Net periodic (expense) benefit cost	(91)	(54)	(7)
Healthcare & Life Ins
Components of net periodic benefit cost:
Service cost	176	166	170
Interest cost	242	321	317
Amortization of gain	89	39	118
Expected return on plan assets	0	0	0
Amortization of prior service credit	(241)	(241)	(241)
Net periodic (expense) benefit cost	266	285	364
Healthcare & Life Ins | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	176	166	170
Interest cost	242	321	317
Amortization of gain	89	39	118
Expected return on plan assets	0	0	0
Amortization of prior service credit	(241)	(241)	(241)
Net periodic (expense) benefit cost	$ 266	$ 285	$ 364