Leases - Operating and Finance Leases (Details) $ in Thousands	12 Months Ended
	Dec. 28, 2019 USD ($)
Lease, Cost [Abstract]
Operating lease cost	$ 10,451
Variable lease cost	423
Short-term lease cost	38,417
Financing lease cost:
Amortization of right-of-use assets	11,062
Interest on lease liabilities	3,171
Total lease cost	63,524
Operating leases:
Operating lease right-of-use assets	32,777
Current operating lease liabilities	8,427
Noncurrent operating lease liabilities	25,381
Total operating lease liabilities	33,808
Finance leases:
Property and equipment, gross	82,660
Less accumulated depreciation	(24,907)
Property and equipment, net	57,753
Current finance lease liabilities	16,007
Long-term finance lease liabilities	40,410
Total finance lease liabilities	$ 56,417
Lease Term
Operating leases	8 years 7 months 6 days
Finance lease	2 years 7 months 6 days
Discount Rate
Operating leases	5.50%
Finance lease	5.50%
Operating Leases
2020	$ 9,958
2021	7,960
2022	4,852
2023	3,842
2024	2,246
Thereafter	14,292
Total lease payments	43,150
Less imputed interest	(9,342)
Finance Leases
2020	18,646
2021	21,886
2022	14,940
2023	1,732
2024	2,252
Thereafter	2,829
Total lease payments	62,285
Less imputed interest	(5,868)
Summit Materials, LLC
Lease, Cost [Abstract]
Operating lease cost	10,451
Variable lease cost	423
Short-term lease cost	38,417
Financing lease cost:
Amortization of right-of-use assets	11,062
Interest on lease liabilities	3,171
Total lease cost	63,524
Operating leases:
Operating lease right-of-use assets	32,777
Current operating lease liabilities	8,427
Noncurrent operating lease liabilities	25,381
Total operating lease liabilities	33,808
Finance leases:
Property and equipment, gross	82,660
Less accumulated depreciation	(24,907)
Property and equipment, net	57,753
Current finance lease liabilities	16,007
Long-term finance lease liabilities	40,410
Total finance lease liabilities	$ 56,417
Lease Term
Operating leases	8 years 7 months 6 days
Finance lease	2 years 7 months 6 days
Discount Rate
Operating leases	5.50%
Finance lease	5.50%
Operating Leases
2020	$ 9,958
2021	7,960
2022	4,852
2023	3,842
2024	2,246
Thereafter	14,292
Total lease payments	43,150
Less imputed interest	(9,342)
Finance Leases
2020	18,646
2021	21,886
2022	14,940
2023	1,732
2024	2,252
Thereafter	2,829
Total lease payments	62,285
Less imputed interest	$ (5,868)