GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018
Current assets:
Cash and cash equivalents	$ 288,757			$ 311,319
Accounts receivable, net	309,377			253,256
Cost and estimated earnings in excess of billings	44,001			13,088
Inventories	209,774			204,787
Other current assets	13,632			13,831
Total current assets	865,541			796,281
Property, plant and equipment and finance lease right-of-use asset, net	1,763,066			1,747,449
Goodwill	1,303,086			1,199,699
Intangible assets, net	37,923			23,498
Operating lease right-of-use assets	28,551			32,777
Other assets	52,103			55,519
Total assets	4,292,170			4,067,556
Current liabilities:
Current portion of debt	7,942			7,942
Current portion of acquisition-related liabilities	31,968			32,700
Accounts payable	149,475			116,359
Accrued expenses	144,064			120,005
Current operating lease liabilities	8,193			8,427
Billings in excess of costs and estimated earnings	14,225			13,864
Total current liabilities	355,867			299,297
Long-term debt	1,893,212			1,851,057
Acquisition-related liabilities	12,876			19,801
Noncurrent operating lease liabilities	21,327			25,381
Other noncurrent liabilities	111,435			100,282
Total liabilities	2,722,674			2,622,783
Total liabilities and stockholders’ equity/member's interest	4,292,170			4,067,556
Summit Materials, LLC
Current assets:
Cash and cash equivalents	288,757			311,319
Accounts receivable, net	309,377			253,256
Intercompany receivables	0			0
Cost and estimated earnings in excess of billings	44,001			13,088
Inventories	209,774			204,787
Other current assets	13,632			13,831
Total current assets	865,541			796,281
Property, plant and equipment and finance lease right-of-use asset, net	1,763,066			1,747,449
Goodwill	1,304,086			1,200,699
Intangible assets, net	37,923			23,498
Operating lease right-of-use assets	28,551			32,777
Other assets	52,103			55,519
Total assets	4,051,270			3,856,223
Current liabilities:
Current portion of debt	7,942			7,942
Current portion of acquisition-related liabilities	29,592			30,200
Accounts payable	150,086			116,970
Accrued expenses	144,295			120,237
Current operating lease liabilities	8,193			8,427
Intercompany payables	0			0
Billings in excess of costs and estimated earnings	14,225			13,864
Total current liabilities	354,333			297,640
Long-term debt	1,893,212			1,851,057
Acquisition-related liabilities	12,876			17,666
Noncurrent operating lease liabilities	21,327			25,381
Other noncurrent liabilities	155,011			151,329
Total liabilities	2,436,759			2,343,073
Total member's interest	1,614,511	$ 1,520,828	$ 1,442,973	1,513,150	$ 1,450,664	$ 1,360,044	$ 1,299,730	$ 1,385,431
Total liabilities and stockholders’ equity/member's interest	4,051,270			3,856,223
Summit Materials, LLC | Eliminations
Current assets:
Cash and cash equivalents	(4,588)			(6,477)
Accounts receivable, net	(87)			(33)
Intercompany receivables	(1,553,152)			(1,385,708)
Cost and estimated earnings in excess of billings	0			0
Inventories	0			0
Other current assets	0			0
Total current assets	(1,557,827)			(1,392,218)
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	(4,068,357)			(3,709,690)
Total assets	(5,626,184)			(5,101,908)
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	0			0
Accounts payable	(87)			(33)
Accrued expenses	(4,588)			(6,477)
Current operating lease liabilities	0			0
Intercompany payables	(1,553,152)			(1,385,708)
Billings in excess of costs and estimated earnings	0			0
Total current liabilities	(1,557,827)			(1,392,218)
Long-term debt	0			0
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	0			0
Other noncurrent liabilities	(164,421)			(137,018)
Total liabilities	(1,722,248)			(1,529,236)
Total member's interest	(3,903,936)			(3,572,672)
Total liabilities and stockholders’ equity/member's interest	(5,626,184)			(5,101,908)
Summit Materials, LLC | Issuers | Reportable Legal Entities
Current assets:
Cash and cash equivalents	277,052			302,474
Accounts receivable, net	1			0
Intercompany receivables	424,533			443,323
Cost and estimated earnings in excess of billings	0			0
Inventories	0			0
Other current assets	2,219			1,763
Total current assets	703,805			747,560
Property, plant and equipment and finance lease right-of-use asset, net	10,383			11,602
Goodwill	0			0
Intangible assets, net	0			0
Operating lease right-of-use assets	2,793			3,316
Other assets	3,922,503			3,596,161
Total assets	4,639,484			4,358,639
Current liabilities:
Current portion of debt	7,942			7,942
Current portion of acquisition-related liabilities	0			0
Accounts payable	5,935			4,588
Accrued expenses	37,693			51,043
Current operating lease liabilities	874			764
Intercompany payables	1,072,127			922,356
Billings in excess of costs and estimated earnings	0			0
Total current liabilities	1,124,571			986,693
Long-term debt	1,893,212			1,851,057
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	2,801			3,480
Other noncurrent liabilities	4,389			4,259
Total liabilities	3,024,973			2,845,489
Total member's interest	1,614,511			1,513,150
Total liabilities and stockholders’ equity/member's interest	4,639,484			4,358,639
Summit Materials, LLC | 100% Owned Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	3,350			5,488
Accounts receivable, net	289,977			234,053
Intercompany receivables	1,128,619			942,385
Cost and estimated earnings in excess of billings	40,711			12,291
Inventories	203,217			199,794
Other current assets	9,724			10,308
Total current assets	1,675,598			1,404,319
Property, plant and equipment and finance lease right-of-use asset, net	1,691,382			1,674,443
Goodwill	1,222,786			1,142,063
Intangible assets, net	37,923			23,498
Operating lease right-of-use assets	21,647			24,551
Other assets	197,286			168,314
Total assets	4,846,622			4,437,188
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	29,592			30,200
Accounts payable	135,658			103,812
Accrued expenses	105,201			72,970
Current operating lease liabilities	6,239			6,571
Intercompany payables	469,880			447,827
Billings in excess of costs and estimated earnings	13,139			12,183
Total current liabilities	759,709			673,563
Long-term debt	0			0
Acquisition-related liabilities	12,876			17,666
Noncurrent operating lease liabilities	15,600			18,047
Other noncurrent liabilities	207,881			203,919
Total liabilities	996,066			913,195
Total member's interest	3,850,556			3,523,993
Total liabilities and stockholders’ equity/member's interest	4,846,622			4,437,188
Summit Materials, LLC | Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	12,943			9,834
Accounts receivable, net	19,486			19,236
Intercompany receivables	0			0
Cost and estimated earnings in excess of billings	3,290			797
Inventories	6,557			4,993
Other current assets	1,689			1,760
Total current assets	43,965			36,620
Property, plant and equipment and finance lease right-of-use asset, net	61,301			61,404
Goodwill	81,300			58,636
Intangible assets, net	0			0
Operating lease right-of-use assets	4,111			4,910
Other assets	671			734
Total assets	191,348			162,304
Current liabilities:
Current portion of debt	0			0
Current portion of acquisition-related liabilities	0			0
Accounts payable	8,580			8,603
Accrued expenses	5,989			2,701
Current operating lease liabilities	1,080			1,092
Intercompany payables	11,145			15,525
Billings in excess of costs and estimated earnings	1,086			1,681
Total current liabilities	27,880			29,602
Long-term debt	0			0
Acquisition-related liabilities	0			0
Noncurrent operating lease liabilities	2,926			3,854
Other noncurrent liabilities	107,162			80,169
Total liabilities	137,968			113,625
Total member's interest	53,380			48,679
Total liabilities and stockholders’ equity/member's interest	$ 191,348			$ 162,304