Employee Benefit Plans - Amounts Recognized in Other Comprehensive (Gain) Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Pension Benefits
Amounts recognized in other comprehensive (income) loss:
Net actuarial loss (gain)	$ 1,760	$ (1,300)	$ 1,068
Prior service cost	0	0	0
Amortization of prior year service cost	0	0	0
Curtailment benefit	0	0	(429)
Amortization of gain	(202)	(312)	(547)
Adjustment to plan benefits	0	0	0
Total amount recognized	1,558	(1,612)	92
Pension Benefits | Summit Materials, LLC
Amounts recognized in other comprehensive (income) loss:
Net actuarial loss (gain)	1,760	(1,300)	1,068
Prior service cost	0	0	0
Amortization of prior year service cost	0	0	0
Curtailment benefit	0	0	(429)
Amortization of gain	(202)	(312)	(547)
Adjustment to plan benefits	0	0	0
Total amount recognized	1,558	(1,612)	92
Healthcare & Life Ins
Amounts recognized in other comprehensive (income) loss:
Net actuarial loss (gain)	165	(172)	(338)
Prior service cost	0	0	(572)
Amortization of prior year service cost	241	241	168
Curtailment benefit	0	0	0
Amortization of gain	(39)	(118)	(64)
Adjustment to plan benefits	0	0	(414)
Total amount recognized	367	(49)	(1,220)
Healthcare & Life Ins | Summit Materials, LLC
Amounts recognized in other comprehensive (income) loss:
Net actuarial loss (gain)	165	(172)	(338)
Prior service cost	0	0	(572)
Amortization of prior year service cost	241	241	168
Curtailment benefit	0	0	0
Amortization of gain	(39)	(118)	(64)
Adjustment to plan benefits	0	0	(414)
Total amount recognized	$ 367	$ (49)	$ (1,220)