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. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Condensed Consolidating Statements of Operations
Revenue	$ 732,082	$ 694,661	$ 1,665,612	$ 1,609,972
Cost of revenue (excluding items shown separately below)	482,979	471,803	1,155,476	1,130,596
Depreciation, depletion, amortization and accretion	55,127	53,974	164,140	150,663
Operating income	130,881	108,167	153,632	133,921
Gain on sale of business	0	(12,108)	0	(12,108)
Income from operations before taxes	103,839	94,757	58,998	71,206
Income tax expense	45,602	20,765	34,272	16,249
Net income attributable to Summit Inc.	55,757	71,289	23,395	53,069
Comprehensive income attributable to member of Summit Materials, LLC	54,904	72,778	25,652	51,817
Summit Materials, LLC
Condensed Consolidating Statements of Operations
Revenue	732,082	694,661	1,665,612	1,609,972
Cost of revenue (excluding items shown separately below)	482,979	471,803	1,155,476	1,130,596
General and administrative expenses	63,095	60,717	192,364	194,792
Depreciation, depletion, amortization and accretion	55,127	53,974	164,140	150,663
Operating income	130,881	108,167	153,632	133,921
Other (income) loss, net	(1,875)	(3,371)	6,211	(11,793)
Interest expense (income)	28,800	28,720	88,020	86,066
Gain on sale of business	0	(12,108)	0	(12,108)
Income from operations before taxes	103,956	94,926	59,401	71,756
Income tax expense	18,757	4,499	12,265	3,323
Net income attributable to Summit Inc.	85,199	90,427	47,136	68,433
Comprehensive income attributable to member of Summit Materials, LLC	84,026	92,484	50,251	66,697
Summit Materials, LLC | Eliminations
Condensed Consolidating Statements of Operations
Revenue	(4,103)	(1,548)	(8,047)	(4,585)
Cost of revenue (excluding items shown separately below)	(4,103)	(1,548)	(8,047)	(4,585)
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	131,665	132,266	197,162	203,534
Interest expense (income)	0	0	0	0
Gain on sale of business		0		0
Income from operations before taxes	(131,665)	(132,266)	(197,162)	(203,534)
Income tax expense	0	0	0	0
Net income attributable to Summit Inc.	(131,665)	(132,266)	(197,162)	(203,534)
Comprehensive income attributable to member of Summit Materials, LLC	(132,838)	(130,209)	(194,047)	(205,270)
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	13,603	10,951	37,887	45,175
Depreciation, depletion, amortization and accretion	1,042	574	2,986	1,919
Operating income	(14,645)	(11,525)	(40,873)	(47,094)
Other (income) loss, net	(132,261)	(132,382)	(183,971)	(204,677)
Interest expense (income)	32,129	29,396	94,848	87,924
Gain on sale of business		0		0
Income from operations before taxes	85,487	91,461	48,250	69,659
Income tax expense	288	1,034	1,114	1,226
Net income attributable to Summit Inc.	85,199	90,427	47,136	68,433
Comprehensive income attributable to member of Summit Materials, LLC	84,026	92,484	50,251	66,697
100% Owned Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	706,999	668,145	1,603,338	1,548,960
Cost of revenue (excluding items shown separately below)	467,595	452,394	1,114,401	1,087,150
General and administrative expenses	46,816	46,910	146,136	141,372
Depreciation, depletion, amortization and accretion	52,739	52,213	156,827	145,135
Operating income	139,849	116,628	185,974	175,303
Other (income) loss, net	(1,501)	(3,019)	(6,427)	(10,563)
Interest expense (income)	(4,532)	(1,882)	(10,443)	(5,447)
Gain on sale of business		(12,108)		(12,108)
Income from operations before taxes	145,882	133,637	202,844	203,421
Income tax expense	17,325	2,898	9,673	1,481
Net income attributable to Summit Inc.	128,557	130,739	193,171	201,940
Comprehensive income attributable to member of Summit Materials, LLC	128,402	130,652	193,319	200,497
Non-Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	29,186	28,064	70,321	65,597
Cost of revenue (excluding items shown separately below)	19,487	20,957	49,122	48,031
General and administrative expenses	2,676	2,856	8,341	8,245
Depreciation, depletion, amortization and accretion	1,346	1,187	4,327	3,609
Operating income	5,677	3,064	8,531	5,712
Other (income) loss, net	222	(236)	(553)	(87)
Interest expense (income)	1,203	1,206	3,615	3,589
Gain on sale of business		0		0
Income from operations before taxes	4,252	2,094	5,469	2,210
Income tax expense	1,144	567	1,478	616
Net income attributable to Summit Inc.	3,108	1,527	3,991	1,594
Comprehensive income attributable to member of Summit Materials, LLC	$ 4,436	$ (443)	$ 728	$ 4,773