GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Condensed Consolidating Statements of Operations
Revenue	$ 709,619	$ 732,082	$ 1,707,818	$ 1,665,612
Cost of revenue (excluding items shown separately below)	469,004	482,979	1,165,317	1,155,476
Depreciation, depletion, amortization and accretion	58,054	55,127	163,760	164,140
Operating income	100,617	130,881	158,957	153,632
Income from operations before taxes	73,156	103,839	79,597	58,998
Income tax expense (benefit)	(19,613)	45,602	(25,333)	34,272
Net income (loss) attributable to Summit Inc/LLC	90,730	55,757	102,815	23,395
Comprehensive loss attributable to member of Summit Materials, LLC	92,202	54,904	100,342	25,652
Summit Materials, LLC
Condensed Consolidating Statements of Operations
Revenue	709,619	732,082	1,707,818	1,665,612
Cost of revenue (excluding items shown separately below)	469,004	482,979	1,165,317	1,155,476
General and administrative expenses	81,944	63,095	219,784	192,364
Depreciation, depletion, amortization and accretion	58,054	55,127	163,760	164,140
Operating income	100,617	130,881	158,957	153,632
Other (income) loss, net	2,838	(1,875)	1,311	6,211
Interest expense (income)	24,561	28,800	77,807	88,020
Income from operations before taxes	73,218	103,956	79,839	59,401
Income tax expense (benefit)	(5,106)	18,757	(4,877)	12,265
Net income (loss) attributable to Summit Inc/LLC	78,324	85,199	84,716	47,136
Comprehensive loss attributable to member of Summit Materials, LLC	80,342	84,026	81,321	50,251
Summit Materials, LLC | Eliminations
Condensed Consolidating Statements of Operations
Revenue	(4,003)	(4,103)	(13,264)	(8,047)
Cost of revenue (excluding items shown separately below)	(4,003)	(4,103)	(13,264)	(8,047)
General and administrative expenses	0	0	0	0
Depreciation, depletion, amortization and accretion	0	0	0	0
Operating income	0	0	0	0
Other (income) loss, net	138,675	131,665	237,774	197,162
Interest expense (income)	0	0	0	0
Income from operations before taxes	(138,675)	(131,665)	(237,774)	(197,162)
Income tax expense (benefit)	0	0	0	0
Net income (loss) attributable to Summit Inc/LLC	(138,675)	(131,665)	(237,774)	(197,162)
Comprehensive loss attributable to member of Summit Materials, LLC	(136,657)	(132,838)	(241,169)	(194,047)
Issuers | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	0	0	0	0
Cost of revenue (excluding items shown separately below)	0	0	0	0
General and administrative expenses	23,955	13,603	50,964	37,887
Depreciation, depletion, amortization and accretion	980	1,042	2,960	2,986
Operating income	(24,935)	(14,645)	(53,924)	(40,873)
Other (income) loss, net	(134,672)	(132,261)	(235,001)	(183,971)
Interest expense (income)	31,170	32,129	95,379	94,848
Income from operations before taxes	78,567	85,487	85,698	48,250
Income tax expense (benefit)	243	288	982	1,114
Net income (loss) attributable to Summit Inc/LLC	78,324	85,199	84,716	47,136
Comprehensive loss attributable to member of Summit Materials, LLC	80,342	84,026	81,321	50,251
100% Owned Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	689,842	706,999	1,659,526	1,603,338
Cost of revenue (excluding items shown separately below)	458,028	467,595	1,138,689	1,114,401
General and administrative expenses	55,242	46,816	160,670	146,136
Depreciation, depletion, amortization and accretion	55,542	52,739	156,697	156,827
Operating income	121,030	139,849	203,470	185,974
Other (income) loss, net	(889)	(1,501)	(1,733)	(6,427)
Interest expense (income)	(7,990)	(4,532)	(21,352)	(10,443)
Income from operations before taxes	129,909	145,882	226,555	202,844
Income tax expense (benefit)	(6,286)	17,325	(7,383)	9,673
Net income (loss) attributable to Summit Inc/LLC	136,195	128,557	233,938	193,171
Comprehensive loss attributable to member of Summit Materials, LLC	136,195	128,402	233,938	193,319
Non-Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	23,780	29,186	61,556	70,321
Cost of revenue (excluding items shown separately below)	14,979	19,487	39,892	49,122
General and administrative expenses	2,747	2,676	8,150	8,341
Depreciation, depletion, amortization and accretion	1,532	1,346	4,103	4,327
Operating income	4,522	5,677	9,411	8,531
Other (income) loss, net	(276)	222	271	(553)
Interest expense (income)	1,381	1,203	3,780	3,615
Income from operations before taxes	3,417	4,252	5,360	5,469
Income tax expense (benefit)	937	1,144	1,524	1,478
Net income (loss) attributable to Summit Inc/LLC	2,480	3,108	3,836	3,991
Comprehensive loss attributable to member of Summit Materials, LLC	$ 462	$ 4,436	$ 7,231	$ 728