GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Oct. 02, 2021	Jul. 03, 2021	Apr. 03, 2021	Jan. 02, 2021	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019
Current assets:
Cash and cash equivalents	$ 258,083			$ 418,181
Accounts receivable, net	333,351			254,696
Cost and estimated earnings in excess of billings	34,181			8,666
Inventories	195,312			200,308
Other current assets	13,855			11,428
Total current assets	834,782			893,279
Property, plant and equipment and finance lease right-of-use asset, net	1,841,139			1,850,169
Goodwill	1,174,855			1,201,291
Intangible assets, net	70,338			47,852
Operating lease right-of-use assets	27,339			28,543
Other assets	57,807			55,000
Total assets	4,216,649			4,308,011
Current liabilities:
Current portion of debt	6,354			6,354
Current portion of acquisition-related liabilities	12,809			10,265
Accounts payable	155,919			120,813
Accrued expenses	137,378			160,570
Current operating lease liabilities	6,818			8,188
Billings in excess of costs and estimated earnings	11,631			16,499
Total current liabilities	330,909			322,689
Long-term debt	1,591,989			1,892,347
Acquisition-related liabilities	33,223			12,246
Noncurrent operating lease liabilities	21,596			21,500
Other noncurrent liabilities	136,297			121,281
Total liabilities	2,444,885			2,691,743
Total liabilities and stockholders’ equity/member's interest	4,216,649			4,308,011
Summit Materials, LLC
Current assets:
Cash and cash equivalents	258,083			418,181
Accounts receivable, net	333,351			254,696
Intercompany receivables	0			0
Cost and estimated earnings in excess of billings	34,181			8,666
Inventories	195,312			200,308
Other current assets	13,855			11,428
Total current assets	834,782			893,279
Property, plant and equipment and finance lease right-of-use asset, net	1,841,139			1,850,169
Goodwill	1,175,855			1,202,291
Intangible assets, net	70,338			47,852
Operating lease right-of-use assets	27,339			28,543
Other assets	57,807			55,000
Total assets	4,007,260			4,077,134
Current liabilities:
Current portion of debt	6,354			6,354
Current portion of acquisition-related liabilities	12,809			7,827
Accounts payable	156,530			121,422
Accrued expenses	137,609			160,801
Current operating lease liabilities	6,818			8,188
Intercompany payables	0			0
Billings in excess of costs and estimated earnings	11,631			16,499
Total current liabilities	331,751			321,091
Long-term debt	1,591,989			1,892,347
Acquisition-related liabilities	33,223			12,246
Noncurrent operating lease liabilities	21,596			21,500
Other noncurrent liabilities	190,511			167,182
Total liabilities	2,169,070			2,414,366
Total member's interest	1,838,190	$ 1,748,591	$ 1,655,471	1,662,768	$ 1,614,511	$ 1,520,828	$ 1,442,973	$ 1,513,150
Total liabilities and stockholders’ equity/member's interest	4,007,260			4,077,134
Summit Materials, LLC | Eliminations
Current assets:
Cash and cash equivalents	(2,680)			(3,641)
Accounts receivable, net	(149)			109
Intercompany receivables	(1,962,962)			(1,707,752)
Cost and estimated earnings in excess of billings	0			0
Inventories	0			0
Other current assets	0			0
Total current assets	(1,965,791)			(1,711,284)
Property, plant and equipment and finance lease right-of-use asset, net	0			0
Goodwill	0			0
Intangible assets, net	0			0
Operating lease right-of-use assets	0			0
Other assets	(4,482,602)			(4,175,921)
Total assets	(6,448,393)			(5,887,205)
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	0			0
Accounts payable	(149)			109
Accrued expenses	(2,680)			(3,641)
Current operating lease liabilities	0			0
Intercompany payables	(1,962,962)			(1,707,752)
Billings in excess of costs and estimated earnings	0			0
Total current liabilities	(1,965,791)			(1,711,284)
Long-term debt	0			0
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	0			0
Other noncurrent liabilities	(164,421)			(164,421)
Total liabilities	(2,130,212)			(1,875,705)
Total member's interest	(4,318,181)			(4,011,500)
Total liabilities and stockholders’ equity/member's interest	(6,448,393)			(5,887,205)
Summit Materials, LLC | Issuers | Reportable Legal Entities
Current assets:
Cash and cash equivalents	241,285			401,074
Accounts receivable, net	63			4
Intercompany receivables	375,735			404,459
Cost and estimated earnings in excess of billings	0			0
Inventories	0			0
Other current assets	3,272			2,840
Total current assets	620,355			808,377
Property, plant and equipment and finance lease right-of-use asset, net	6,382			9,410
Goodwill	0			0
Intangible assets, net	0			0
Operating lease right-of-use assets	1,908			2,615
Other assets	4,322,784			4,022,729
Total assets	4,951,429			4,843,131
Current liabilities:
Current portion of debt	6,354			6,354
Current portion of acquisition-related liabilities	0			0
Accounts payable	5,054			3,889
Accrued expenses	39,884			54,108
Current operating lease liabilities	967			913
Intercompany payables	1,462,313			1,215,043
Billings in excess of costs and estimated earnings	0			0
Total current liabilities	1,514,572			1,280,307
Long-term debt	1,591,989			1,892,347
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	1,832			2,567
Other noncurrent liabilities	4,846			5,142
Total liabilities	3,113,239			3,180,363
Total member's interest	1,838,190			1,662,768
Total liabilities and stockholders’ equity/member's interest	4,951,429			4,843,131
Summit Materials, LLC | 100% Owned Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	1,818			10,287
Accounts receivable, net	309,930			230,199
Intercompany receivables	1,587,227			1,303,293
Cost and estimated earnings in excess of billings	32,632			7,504
Inventories	189,170			193,417
Other current assets	9,799			6,797
Total current assets	2,130,576			1,751,497
Property, plant and equipment and finance lease right-of-use asset, net	1,744,803			1,746,045
Goodwill	1,115,340			1,142,083
Intangible assets, net	65,358			47,852
Operating lease right-of-use assets	20,381			21,880
Other assets	216,523			207,699
Total assets	5,292,981			4,917,056
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	12,809			7,827
Accounts payable	138,423			108,805
Accrued expenses	98,013			106,320
Current operating lease liabilities	5,153			6,114
Intercompany payables	498,077			485,401
Billings in excess of costs and estimated earnings	10,852			15,508
Total current liabilities	763,327			729,975
Long-term debt	0			0
Acquisition-related liabilities	33,223			12,246
Noncurrent operating lease liabilities	15,562			16,062
Other noncurrent liabilities	230,981			208,540
Total liabilities	1,043,093			966,823
Total member's interest	4,249,888			3,950,233
Total liabilities and stockholders’ equity/member's interest	5,292,981			4,917,056
Summit Materials, LLC | Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	17,660			10,461
Accounts receivable, net	23,507			24,384
Intercompany receivables	0			0
Cost and estimated earnings in excess of billings	1,549			1,162
Inventories	6,142			6,891
Other current assets	784			1,791
Total current assets	49,642			44,689
Property, plant and equipment and finance lease right-of-use asset, net	89,954			94,714
Goodwill	60,515			60,208
Intangible assets, net	4,980			0
Operating lease right-of-use assets	5,050			4,048
Other assets	1,102			493
Total assets	211,243			204,152
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	0			0
Accounts payable	13,202			8,619
Accrued expenses	2,392			4,014
Current operating lease liabilities	698			1,161
Intercompany payables	2,572			7,308
Billings in excess of costs and estimated earnings	779			991
Total current liabilities	19,643			22,093
Long-term debt	0			0
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	4,202			2,871
Other noncurrent liabilities	119,105			117,921
Total liabilities	142,950			142,885
Total member's interest	68,293			61,267
Total liabilities and stockholders’ equity/member's interest	$ 211,243			$ 204,152