ACQUISITIONS, GOODWILL AND INTANGIBLES - Intangible Assets By Type (Details) - USD ($) $ in Thousands	Sep. 26, 2020	Dec. 28, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 50,390	$ 33,864
Accumulated Amortization	(12,467)	(10,366)
Net Carrying Amount	37,923	23,498
Operating permits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	23,345	6,609
Accumulated Amortization	(1,162)	(290)
Net Carrying Amount	22,183	6,319
Mineral leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	19,225	19,064
Accumulated Amortization	(7,286)	(6,408)
Net Carrying Amount	11,939	12,656
Reserve rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,234	6,234
Accumulated Amortization	(2,444)	(2,248)
Net Carrying Amount	3,790	3,986
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,000	1,000
Accumulated Amortization	(1,000)	(958)
Net Carrying Amount	0	42
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	586	957
Accumulated Amortization	(575)	(462)
Net Carrying Amount	11	495
Summit Materials, LLC
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	50,390	33,864
Accumulated Amortization	(12,467)	(10,366)
Net Carrying Amount	37,923	23,498
Summit Materials, LLC | Operating permits
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	23,345	6,609
Accumulated Amortization	(1,162)	(290)
Net Carrying Amount	22,183	6,319
Summit Materials, LLC | Mineral leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	19,225	19,064
Accumulated Amortization	(7,286)	(6,408)
Net Carrying Amount	11,939	12,656
Summit Materials, LLC | Reserve rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,234	6,234
Accumulated Amortization	(2,444)	(2,248)
Net Carrying Amount	3,790	3,986
Summit Materials, LLC | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,000	1,000
Accumulated Amortization	(1,000)	(958)
Net Carrying Amount	0	42
Summit Materials, LLC | Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	586	957
Accumulated Amortization	(575)	(462)
Net Carrying Amount	$ 11	$ 495