Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Summit Materials, LLC
Components of net periodic benefit cost:
Service cost			$ 279
Interest cost			1,049
Amortization of gain			463
Expected return on plan assets			(1,386)
Curtailments			0
Net periodic (expense) benefit cost			405
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 67	$ 285	279
Interest cost	898	998	1,049
Amortization of gain	312	547	463
Expected return on plan assets	(1,284)	(1,302)	(1,386)
Curtailments	0	0	0
Amortization of prior service credit	0	0	0
Net periodic (expense) benefit cost	(7)	528	405
Pension Benefits | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	67	285
Interest cost	898	998
Amortization of gain	312	547
Expected return on plan assets	(1,284)	(1,302)
Curtailments	0	0
Amortization of prior service credit	0	0	0
Net periodic (expense) benefit cost	(7)	528
Healthcare & Life Ins
Components of net periodic benefit cost:
Service cost	170	184	230
Interest cost	317	365	470
Amortization of gain	118	64	207
Expected return on plan assets	0	0	0
Curtailments	0	0	0
Amortization of prior service credit	(241)	(168)	(174)
Net periodic (expense) benefit cost	364	445	733
Healthcare & Life Ins | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	170	184	230
Interest cost	317	365	470
Amortization of gain	118	64	207
Expected return on plan assets	0	0	0
Curtailments	0	0	0
Amortization of prior service credit	(241)	(168)	(174)
Net periodic (expense) benefit cost	$ 364	$ 445	$ 733