Employee Benefit Plans - Obligations and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Change in fair value of plan assets:
Beginning of period	$ 18,242
End of period	19,058	$ 18,242
Summit Materials, LLC
Change in fair value of plan assets:
Beginning of period	18,242
End of period	19,058	18,242
Pension Benefits
Change in benefit obligations:
Beginning of period	26,181	24,203
Service cost	71	60	$ 67
Interest cost	733	928	898
Actuarial (gain) loss	2,425	2,571
Benefits paid	(1,583)	(1,581)
End of period	27,827	26,181	24,203
Change in fair value of plan assets:
Beginning of period	18,242	17,449
Actual return on plan assets	1,916	2,055
Employer contributions	483	319
Benefits paid	(1,583)	(1,581)
End of period	19,058	18,242	17,449
Funded status of plans	(8,769)	(7,939)
Current liabilities	0	0
Noncurrent liabilities	(8,769)	(7,939)
Liability recognized	(8,769)	(7,939)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	10,689	9,286
Prior service cost	0	0
Total amount recognized	10,689	9,286
Pension Benefits | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	26,181	24,203
Service cost	71	60	67
Interest cost	733	928	898
Actuarial (gain) loss	2,425	2,571
Benefits paid	(1,583)	(1,581)
End of period	27,827	26,181	24,203
Change in fair value of plan assets:
Beginning of period	18,242	17,449
Actual return on plan assets	1,916	2,055
Employer contributions	483	319
Benefits paid	(1,583)	(1,581)
End of period	19,058	18,242	17,449
Funded status of plans	(8,769)	(7,939)
Current liabilities	0	0
Noncurrent liabilities	(8,769)	(7,939)
Liability recognized	(8,769)	(7,939)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	10,689	9,286
Prior service cost	0	0
Total amount recognized	10,689	9,286
Healthcare & Life Ins
Change in benefit obligations:
Beginning of period	9,090	9,203
Service cost	176	166	170
Interest cost	242	321	317
Actuarial (gain) loss	676	165
Benefits paid	(955)	(765)
End of period	9,229	9,090	9,203
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	955	765
Benefits paid	(955)	(765)
End of period	0	0	0
Funded status of plans	(9,229)	(9,090)
Current liabilities	(636)	(653)
Noncurrent liabilities	(8,593)	(8,437)
Liability recognized	(9,229)	(9,090)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	2,707	2,121
Prior service cost	(1,690)	(1,931)
Total amount recognized	1,017	190
Healthcare & Life Ins | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	9,090	9,203
Service cost	176	166	170
Interest cost	242	321	317
Actuarial (gain) loss	676	165
Benefits paid	(955)	(765)
End of period	9,229	9,090	9,203
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	955	765
Benefits paid	(955)	(765)
End of period	0	0	$ 0
Funded status of plans	(9,229)	(9,090)
Current liabilities	(636)	(653)
Noncurrent liabilities	(8,593)	(8,437)
Liability recognized	(9,229)	(9,090)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	2,707	2,121
Prior service cost	(1,690)	(1,931)
Total amount recognized	$ 1,017	$ 190