Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	$ 184
Interest cost	365
Pension Benefits
Components of net periodic benefit cost:
Service cost	285	$ 279	$ 159
Interest cost	998	1,049	1,041
Amortization of gain	547	463	326
Expected return on plan assets	(1,302)	(1,386)	(1,385)
Net periodic benefit cost	528	405	141
Pension Benefits | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	285	279	159
Interest cost	998	1,049	1,041
Amortization of gain	547	463	326
Expected return on plan assets	(1,302)	(1,386)	(1,385)
Net periodic benefit cost	528	405	141
Healthcare & Life Ins
Components of net periodic benefit cost:
Service cost	184	230	149
Interest cost	365	470	447
Amortization of gain	64	207	235
Amortization of prior service credit	(168)	(174)	(174)
Net periodic benefit cost	445	733	657
Healthcare & Life Ins | Summit Materials, LLC
Components of net periodic benefit cost:
Service cost	184	230	149
Interest cost	365	470	447
Amortization of gain	64	207	235
Amortization of prior service credit	(168)	(174)	(174)
Net periodic benefit cost	$ 445	$ 733	$ 657