GOODWILL AND INTANGIBLES - Intangible Assets By Type (Details) - USD ($) $ in Thousands	Sep. 28, 2019	Dec. 29, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 34,112	$ 26,707
Accumulated Amortization	(9,666)	(8,247)
Net Carrying Amount	24,446	18,460
Mineral leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	19,064	19,064
Accumulated Amortization	(6,124)	(5,259)
Net Carrying Amount	12,940	13,805
Reserve rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,234	6,234
Accumulated Amortization	(2,174)	(1,940)
Net Carrying Amount	4,060	4,294
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,000	1,000
Accumulated Amortization	(933)	(858)
Net Carrying Amount	67	142
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,814	409
Accumulated Amortization	(435)	(190)
Net Carrying Amount	7,379	219
Summit Materials, LLC
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34,112	26,707
Accumulated Amortization	(9,666)	(8,247)
Net Carrying Amount	24,446	18,460
Summit Materials, LLC | Mineral leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	19,064	19,064
Accumulated Amortization	(6,124)	(5,259)
Net Carrying Amount	12,940	13,805
Summit Materials, LLC | Reserve rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,234	6,234
Accumulated Amortization	(2,174)	(1,940)
Net Carrying Amount	4,060	4,294
Summit Materials, LLC | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,000	1,000
Accumulated Amortization	(933)	(858)
Net Carrying Amount	67	142
Summit Materials, LLC | Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,814	409
Accumulated Amortization	(435)	(190)
Net Carrying Amount	$ 7,379	$ 219