LEASES - Operating and Finance Leases (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2021	Sep. 26, 2020	Oct. 02, 2021	Sep. 26, 2020	Jan. 02, 2021
Lease, Cost [Abstract]
Operating lease cost	$ 1,755	$ 2,478	$ 5,175	$ 7,576
Variable lease cost	90	90	263	253
Short-term lease cost	12,373	14,335	30,517	33,369
Financing lease cost:
Amortization of right-of-use assets	2,168	3,439	7,835	9,307
Interest on lease liabilities	475	780	1,689	2,329
Total lease cost	16,861	21,122	45,479	52,834
Operating leases:
Operating lease right-of-use assets	27,339		27,339		$ 28,543
Current operating lease liabilities	6,818		6,818		8,188
Noncurrent operating lease liabilities	21,596		21,596		21,500
Total operating lease liabilities	28,414		28,414		29,688
Finance leases:
Property and equipment, gross	69,115		69,115		92,679
Less accumulated depreciation	$ (29,387)		$ (29,387)		$ (32,828)
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property, plant and equipment and finance lease right-of-use asset, net		Property, plant and equipment and finance lease right-of-use asset, net		Property, plant and equipment and finance lease right-of-use asset, net
Property and equipment, net	$ 39,728		$ 39,728		$ 59,851
Current finance lease liabilities	17,424		17,424		24,601
Long-term finance lease liabilities	18,249		18,249		31,727
Total finance lease liabilities	$ 35,673		$ 35,673		$ 56,328
Weighted average remaining lease term (years):
Operating leases	9 years 1 month 6 days		9 years 1 month 6 days		8 years 8 months 12 days
Finance lease	2 years 6 months		2 years 6 months		2 years 4 months 24 days
Weighted average discount rate:
Operating leases	5.00%		5.00%		5.30%
Finance leases	5.30%		5.30%		5.20%
Operating Leases
2021 (three months)	$ 2,103		$ 2,103
2022	7,489		7,489
2023	5,830		5,830
2024	3,662		3,662
2025	2,444		2,444
2026	1,794		1,794
Thereafter	12,976		12,976
Total lease payments	36,298		36,298
Less imputed interest	(7,884)		(7,884)
Present value of lease payments	28,414		28,414		$ 29,688
Finance Leases
2021 (three months)	3,950		3,950
2022	18,668		18,668
2023	7,386		7,386
2024	3,209		3,209
2025	2,583		2,583
2026	990		990
Thereafter	1,843		1,843
Total lease payments	38,629		38,629
Less imputed interest	(2,956)		(2,956)
Present value of lease payments	35,673		35,673		56,328
Summit Materials, LLC
Lease, Cost [Abstract]
Operating lease cost	1,755	2,478	5,175	7,576
Variable lease cost	90	90	263	253
Short-term lease cost	12,373	14,335	30,517	33,369
Financing lease cost:
Amortization of right-of-use assets	2,168	3,439	7,835	9,307
Interest on lease liabilities	475	780	1,689	2,329
Total lease cost	16,861	$ 21,122	45,479	$ 52,834
Operating leases:
Operating lease right-of-use assets	27,339		27,339		28,543
Current operating lease liabilities	6,818		6,818		8,188
Noncurrent operating lease liabilities	21,596		21,596		21,500
Total operating lease liabilities	28,414		28,414		29,688
Finance leases:
Property and equipment, gross	69,115		69,115		92,679
Less accumulated depreciation	$ (29,387)		$ (29,387)		$ (32,828)
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property, plant and equipment and finance lease right-of-use asset, net		Property, plant and equipment and finance lease right-of-use asset, net		Property, plant and equipment and finance lease right-of-use asset, net
Property and equipment, net	$ 39,728		$ 39,728		$ 59,851
Current finance lease liabilities	17,424		17,424		24,601
Long-term finance lease liabilities	18,249		18,249		31,727
Total finance lease liabilities	$ 35,673		$ 35,673		$ 56,328
Weighted average remaining lease term (years):
Operating leases	9 years 1 month 6 days		9 years 1 month 6 days		8 years 8 months 12 days
Finance lease	2 years 6 months		2 years 6 months		2 years 4 months 24 days
Weighted average discount rate:
Operating leases	5.00%		5.00%		5.30%
Finance leases	5.30%		5.30%		5.20%
Operating Leases
2021 (three months)	$ 2,103		$ 2,103
2022	7,489		7,489
2023	5,830		5,830
2024	3,662		3,662
2025	2,444		2,444
2026	1,794		1,794
Thereafter	12,976		12,976
Total lease payments	36,298		36,298
Less imputed interest	(7,884)		(7,884)
Present value of lease payments	28,414		28,414		$ 29,688
Finance Leases
2021 (three months)	3,950		3,950
2022	18,668		18,668
2023	7,386		7,386
2024	3,209		3,209
2025	2,583		2,583
2026	990		990
Thereafter	1,843		1,843
Total lease payments	38,629		38,629
Less imputed interest	(2,956)		(2,956)
Present value of lease payments	$ 35,673		$ 35,673		$ 56,328