Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Condensed Financial Statements, Captions [Line Items]
Revenue	$ 2,332,451	$ 2,222,140	$ 2,101,002
Cost of revenue	1,583,996	1,526,332	1,475,779
Depreciation, depletion, amortization and accretion	221,320	217,102	204,910
Operating income	225,173	213,558	162,466
Gain on sale of business	0	0	(12,108)
Income from operations before taxes	129,055	78,224	96,077
Income tax expense (benefit)	(12,185)	17,101	59,747
Net income attributable to Summit Inc.	137,967	59,066	33,906
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	139,722	59,833	29,201
Summit Materials, LLC
Condensed Financial Statements, Captions [Line Items]
Revenue	2,332,451	2,222,140	2,101,002
Cost of revenue	1,583,996	1,526,332	1,475,779
General and administrative expenses	301,962	265,148	257,847
Depreciation, depletion, amortization and accretion	221,320	217,102	204,910
Operating income	225,173	213,558	162,466
Other (income) loss, net	82	2,588	(15,367)
Interest expense (income)	103,291	115,988	115,831
Gain on sale of business	0	0	(12,108)
Income from operations before taxes	121,800	94,982	74,110
Income tax expense (benefit)	1,063	6,385	10,273
Net income attributable to Summit Inc.	120,737	88,597	63,837
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	123,125	91,242	57,356
Summit Materials, LLC | Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(17,166)	(12,196)	(6,084)
Cost of revenue	(17,166)	(12,196)	(6,084)
General and administrative expenses	0	0	0
Depreciation, depletion, amortization and accretion	0	0	0
Operating income	0	0	0
Other (income) loss, net	331,667	291,662	247,657
Interest expense (income)	0	0	0
Gain on sale of business			0
Income from operations before taxes	(331,667)	(291,662)	(247,657)
Income tax expense (benefit)	0	0	0
Net income attributable to Summit Inc.	(331,667)	(291,662)	(247,657)
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	(329,279)	(289,017)	(254,138)
Summit Materials, LLC | Issuers | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenue	0	0	0
Cost of revenue	0	0	0
General and administrative expenses	72,504	58,099	62,376
Depreciation, depletion, amortization and accretion	3,983	3,997	2,622
Operating income	(76,487)	(62,096)	(64,998)
Other (income) loss, net	(328,914)	(279,517)	(249,204)
Interest expense (income)	130,176	127,734	118,857
Gain on sale of business			0
Income from operations before taxes	122,251	89,687	65,349
Income tax expense (benefit)	1,514	1,090	1,512
Net income attributable to Summit Inc.	120,737	88,597	63,837
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	123,125	91,242	57,356
Summit Materials, LLC | 100% Owned Guarantors | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenue	2,259,865	2,139,457	2,018,428
Cost of revenue	1,542,586	1,473,124	1,416,222
General and administrative expenses	217,897	195,683	184,917
Depreciation, depletion, amortization and accretion	210,038	207,277	197,406
Operating income	289,344	263,373	219,883
Other (income) loss, net	(2,473)	(8,767)	(14,643)
Interest expense (income)	(31,402)	(16,561)	(7,818)
Gain on sale of business			(12,108)
Income from operations before taxes	323,219	288,701	254,452
Income tax expense (benefit)	(4,737)	3,377	8,226
Net income attributable to Summit Inc.	327,956	285,324	246,226
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	330,185	287,395	243,359
Summit Materials, LLC | Non-Guarantors | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenue	89,752	94,879	88,658
Cost of revenue	58,576	65,404	65,641
General and administrative expenses	11,561	11,366	10,554
Depreciation, depletion, amortization and accretion	7,299	5,828	4,882
Operating income	12,316	12,281	7,581
Other (income) loss, net	(198)	(790)	823
Interest expense (income)	4,517	4,815	4,792
Gain on sale of business			0
Income from operations before taxes	7,997	8,256	1,966
Income tax expense (benefit)	4,286	1,918	535
Net income attributable to Summit Inc.	3,711	6,338	1,431
Comprehensive income (loss) income attributable to member of Summit Materials, LLC	$ (906)	$ 1,622	$ 10,779