Employee Benefit Plans - Obligations and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Change in fair value of plan assets:
Beginning of period	$ 19,012
End of period	17,449	$ 19,012
Summit Materials, LLC
Change in benefit obligations:
Service cost			$ 279
Interest cost			1,049
Change in fair value of plan assets:
Beginning of period	19,012
End of period	17,449	19,012
Pension Benefits
Change in benefit obligations:
Beginning of period	27,984	27,608
Service cost	67	285	279
Interest cost	898	998	1,049
Actuarial (gain) loss	(3,136)	1,182
Curtailments	0	(430)
Change in plan provision	0	0
Benefits paid	(1,610)	(1,659)
End of period	24,203	27,984	27,608
Change in fair value of plan assets:
Beginning of period	19,012	18,395
Actual return on plan assets	(551)	1,415
Employer contributions	598	861
Benefits paid	(1,610)	(1,659)
End of period	17,449	19,012	18,395
Funded status of plans	(6,754)	(8,972)
Current liabilities	0	0
Noncurrent liabilities	(6,754)	(8,972)
Liability recognized	(6,754)	(8,972)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	(1,300)	9,341
Prior service cost	(312)	0
Total amount recognized	(1,612)	9,341
Pension Benefits | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	27,984	27,608
Service cost	67	285
Interest cost	898	998
Actuarial (gain) loss	(3,136)	1,182
Curtailments	0
Change in plan provision	0	0
Benefits paid	(1,610)	(1,659)
End of period	24,203	27,984	27,608
Change in fair value of plan assets:
Beginning of period	19,012	18,395
Actual return on plan assets	(551)	1,415
Employer contributions	598	861
Benefits paid	(1,610)	(1,659)
End of period	17,449	19,012	18,395
Funded status of plans	(6,754)	(8,972)
Current liabilities	0	0
Noncurrent liabilities	(6,754)	(8,972)
Liability recognized	(6,754)	(8,972)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	(1,300)	9,341
Prior service cost	(312)	0
Total amount recognized	(1,612)	9,341
Healthcare & Life Ins
Change in benefit obligations:
Beginning of period	9,793	12,770
Service cost	170	184	230
Interest cost	317	365	470
Actuarial (gain) loss	(173)	(338)
Curtailments	0	0
Change in plan provision	0	(2,325)
Benefits paid	(904)	(863)
End of period	9,203	9,793	12,770
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	904	863
Benefits paid	(904)	(863)
End of period	0	0	0
Funded status of plans	(9,203)	(9,793)
Current liabilities	0	(702)
Noncurrent liabilities	(9,203)	(9,091)
Liability recognized	(9,203)	(9,793)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	1,995	2,285
Prior service cost	(2,172)	(2,413)
Total amount recognized	(177)	(128)
Healthcare & Life Ins | Summit Materials, LLC
Change in benefit obligations:
Beginning of period	9,793	12,770
Service cost	170	184	230
Interest cost	317	365	470
Actuarial (gain) loss	(173)	(338)
Change in plan provision	0	(2,325)
Benefits paid	(904)	(863)
End of period	9,203	9,793	12,770
Change in fair value of plan assets:
Beginning of period	0	0
Actual return on plan assets	0	0
Employer contributions	904	863
Benefits paid	(904)	(863)
End of period	0	0	$ 0
Funded status of plans	(9,203)	(9,793)
Current liabilities	0	(702)
Noncurrent liabilities	(9,203)	(9,091)
Liability recognized	(9,203)	(9,793)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain) loss	1,995	2,285
Prior service cost	(2,172)	(2,413)
Total amount recognized	$ (177)	$ (128)