Employee Benefit Plans - Obligations and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Change in fair value of plan assets:
Beginning of period	$ 17,449
End of period	18,242	$ 17,449
Summit Materials, LLC
Change in fair value of plan assets:
Beginning of period	17,449
End of period	18,242	17,449
Pension Benefits
Change in benefit obligations:
Beginning of period	24,203	27,984
Service cost	60	67	$ 285
Interest cost	928	898	998
Actuarial (gain) loss	2,571	(3,136)
Benefits paid	(1,581)	(1,610)
End of period	26,181	24,203	27,984
Change in fair value of plan assets:
Beginning of period	17,449	19,012
Actual return on plan assets	2,055	(551)
Employer contributions	319	598
Benefits paid	(1,581)	(1,610)
End of period	18,242	17,449	19,012
Funded status of plans	(7,939)	(6,754)
Current liabilities	0	0
Noncurrent liabilities	(7,939)	(6,754)
Liability recognized	(7,939)	(6,754)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	9,286	7,728
Prior service cost	0	0
Total amount recognized	9,286	7,728
Pension Benefits | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	24,203	27,984
Service cost	60	67	285
Interest cost	928	898	998
Actuarial (gain) loss	2,571	(3,136)
Benefits paid	(1,581)	(1,610)
End of period	26,181	24,203	27,984
Change in fair value of plan assets:
Beginning of period	17,449	19,012
Actual return on plan assets	2,055	(551)
Employer contributions	319	598
Benefits paid	(1,581)	(1,610)
End of period	18,242	17,449	19,012
Funded status of plans	(7,939)	(6,754)
Current liabilities	0	0
Noncurrent liabilities	(7,939)	(6,754)
Liability recognized	(7,939)	(6,754)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	9,286	7,728
Prior service cost	0	0
Total amount recognized	9,286	7,728
Healthcare & Life Ins
Change in benefit obligations:
Beginning of period	9,203	9,793
Service cost	166	170	184
Interest cost	321	317	365
Actuarial (gain) loss	165	(173)
Benefits paid	(765)	(904)
End of period	9,090	9,203	9,793
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	765	904
Benefits paid	(765)	(904)
End of period	0	0	0
Funded status of plans	(9,090)	(9,203)
Current liabilities	(653)	(687)
Noncurrent liabilities	(8,437)	(8,516)
Liability recognized	(9,090)	(9,203)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	2,121	1,995
Prior service cost	(1,931)	(2,172)
Total amount recognized	190	(177)
Healthcare & Life Ins | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	9,203	9,793
Service cost	166	170	184
Interest cost	321	317	365
Actuarial (gain) loss	165	(173)
Benefits paid	(765)	(904)
End of period	9,090	9,203	9,793
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	765	904
Benefits paid	(765)	(904)
End of period	0	0	$ 0
Funded status of plans	(9,090)	(9,203)
Current liabilities	(653)	(687)
Noncurrent liabilities	(8,437)	(8,516)
Liability recognized	(9,090)	(9,203)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	2,121	1,995
Prior service cost	(1,931)	(2,172)
Total amount recognized	$ 190	$ (177)