Leases - Operating and Finance Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019
Lease, Cost [Abstract]
Operating lease cost	$ 10,134	$ 10,451
Variable lease cost	316	423
Short-term lease cost	44,066	38,417
Financing lease cost:
Amortization of right-of-use assets	12,598	11,062
Interest on lease liabilities	3,068	3,171
Total lease cost	70,182	63,524
Operating leases:
Operating lease right-of-use assets	28,543	32,777
Current operating lease liabilities	8,188	8,427
Noncurrent operating lease liabilities	21,500	25,381
Total operating lease liabilities	29,688	33,808
Finance leases:
Property and equipment, gross	92,679	82,660
Less accumulated depreciation	$ (32,828)	(24,907)
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Property and equipment, net	$ 59,851	57,753
Current finance lease liabilities	24,601	16,007
Long-term finance lease liabilities	31,727	40,410
Total finance lease liabilities	$ 56,328	$ 56,417
Weighted average remaining lease term (years):
Operating leases	8 years 8 months 12 days	8 years 7 months 6 days
Finance lease	2 years 4 months 24 days	2 years 7 months 6 days
Weighted average discount rate:
Operating leases	5.30%	5.50%
Finance lease	5.20%	5.50%
Operating Leases
2021	$ 9,491
2022	6,088
2023	4,663
2024	2,863
2025	1,781
Thereafter	12,961
Total lease payments	37,847
Less imputed interest	(8,159)
Finance Leases
2021	26,742
2022	18,603
2023	7,053
2024	3,207
2025	2,573
Thereafter	2,831
Total lease payments	61,009
Less imputed interest	(4,681)
Summit Materials, LLC
Lease, Cost [Abstract]
Operating lease cost	10,134	$ 10,451
Variable lease cost	316	423
Short-term lease cost	44,066	38,417
Financing lease cost:
Amortization of right-of-use assets	12,598	11,062
Interest on lease liabilities	3,068	3,171
Total lease cost	70,182	63,524
Operating leases:
Operating lease right-of-use assets	28,543	32,777
Current operating lease liabilities	8,188	8,427
Noncurrent operating lease liabilities	21,500	25,381
Total operating lease liabilities	29,688	33,808
Finance leases:
Property and equipment, gross	92,679	82,660
Less accumulated depreciation	$ (32,828)	(24,907)
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Property and equipment, net	$ 59,851	57,753
Current finance lease liabilities	24,601	16,007
Long-term finance lease liabilities	31,727	40,410
Total finance lease liabilities	$ 56,328	$ 56,417
Weighted average remaining lease term (years):
Operating leases	8 years 8 months 12 days	8 years 7 months 6 days
Finance lease	2 years 4 months 24 days	2 years 7 months 6 days
Weighted average discount rate:
Operating leases	5.30%	5.50%
Finance lease	5.20%	5.50%
Operating Leases
2021	$ 9,491
2022	6,088
2023	4,663
2024	2,863
2025	1,781
Thereafter	12,961
Total lease payments	37,847
Less imputed interest	(8,159)
Finance Leases
2021	26,742
2022	18,603
2023	7,053
2024	3,207
2025	2,573
Thereafter	2,831
Total lease payments	61,009
Less imputed interest	$ (4,681)