LEASES - Operating and Finance Leases (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019	Dec. 28, 2019
Lease, Cost [Abstract]
Operating lease cost	$ 2,607	$ 2,455
Variable lease cost	57	72
Short-term lease cost	8,620	6,581
Financing lease cost:
Amortization of right-of-use assets	2,739	2,623
Interest on lease liabilities	771	755
Total lease cost	14,794	12,486
Operating leases:
Operating lease right-of-use assets	30,527		$ 32,777
Current operating lease liabilities	8,131		8,427
Noncurrent operating lease liabilities	23,280		25,381
Total operating lease liabilities	31,411		33,808
Finance leases:
Property and equipment, gross	87,377		82,660
Less accumulated depreciation	(27,251)		(24,907)
Property and equipment, net	60,126		57,753
Current finance lease liabilities	20,724		16,007
Long-term finance lease liabilities	37,300		40,410
Total finance lease liabilities	$ 58,024		$ 56,417
Weighted average remaining lease term (years):
Operating leases	8 years 7 months 6 days		8 years 7 months 6 days
Finance lease	2 years 6 months		2 years 7 months 6 days
Weighted average discount rate (%):
Operating leases	5.50%		5.50%
Finance lease	5.50%		5.50%
Operating Leases
2020 (nine months)	$ 7,116
2021	8,033
2022	4,920
2023	3,877
2024	2,232
2025	1,414
Thereafter	12,613
Total lease payments	40,205
Less imputed interest	(8,794)
Finance Leases
2020 (nine months)	14,513
2021	23,292
2022	16,428
2023	4,472
2024	2,277
2025	1,104
Thereafter	1,743
Total lease payments	63,829
Less imputed interest	(5,805)
Summit Materials, LLC
Lease, Cost [Abstract]
Operating lease cost	2,607	2,455
Variable lease cost	57	72
Short-term lease cost	8,620	6,581
Financing lease cost:
Amortization of right-of-use assets	2,739	2,623
Interest on lease liabilities	771	755
Total lease cost	14,794	$ 12,486
Operating leases:
Operating lease right-of-use assets	30,527		$ 32,777
Current operating lease liabilities	8,131		8,427
Noncurrent operating lease liabilities	23,280		25,381
Total operating lease liabilities	31,411		33,808
Finance leases:
Property and equipment, gross	87,377		82,660
Less accumulated depreciation	(27,251)		(24,907)
Property and equipment, net	60,126		57,753
Current finance lease liabilities	20,724		16,007
Long-term finance lease liabilities	37,300		40,410
Total finance lease liabilities	$ 58,024		$ 56,417
Weighted average remaining lease term (years):
Operating leases	8 years 7 months 6 days		8 years 7 months 6 days
Finance lease	2 years 6 months		2 years 7 months 6 days
Weighted average discount rate (%):
Operating leases	5.50%		5.50%
Finance lease	5.50%		5.50%
Operating Leases
2020 (nine months)	$ 7,116
2021	8,033
2022	4,920
2023	3,877
2024	2,232
2025	1,414
Thereafter	12,613
Total lease payments	40,205
Less imputed interest	(8,794)
Finance Leases
2020 (nine months)	14,513
2021	23,292
2022	16,428
2023	4,472
2024	2,277
2025	1,104
Thereafter	1,743
Total lease payments	63,829
Less imputed interest	$ (5,805)