GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 30, 2018	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017
Current assets:
Cash and cash equivalents	$ 182,589				$ 128,508
Accounts receivable, net	337,060				214,518
Cost and estimated earnings in excess of billings	49,715				18,602
Inventories	197,015				213,851
Other current assets	12,037				16,061
Total current assets	778,416				591,540
Property, plant and equipment, net	1,762,307				1,780,132
Goodwill	1,198,496				1,192,028
Intangible assets, net	24,446				18,460
Operating lease right-of-use assets	33,045			$ 36,800
Other assets	51,772				50,084
Total assets	4,042,114				3,857,641
Current liabilities:
Current portion of debt	6,354				6,354
Current portion of acquisition-related liabilities	34,398				34,270
Accounts payable	152,232				107,702
Accrued expenses	118,003				100,491
Current operating lease liabilities	8,609
Billings in excess of costs and estimated earnings	12,476				11,840
Total current liabilities	332,072				260,657
Long-term debt	1,853,414				1,807,502
Acquisition-related liabilities	40,662				49,468
Noncurrent operating lease liabilities	25,329
Other noncurrent liabilities	93,761				88,195
Total liabilities	2,655,336				2,515,496
Total liabilities and stockholders’ equity/member's interest	4,042,114				3,857,641
Summit Materials, LLC
Current assets:
Cash and cash equivalents	182,589				128,508
Accounts receivable, net	337,060				214,518
Intercompany receivables	0				0
Cost and estimated earnings in excess of billings	49,715				18,602
Inventories	197,015				213,851
Other current assets	12,037				16,061
Total current assets	778,416				591,540
Property, plant and equipment, net	1,762,307				1,780,132
Goodwill	1,199,496				1,193,028
Intangible assets, net	24,446				18,460
Operating lease right-of-use assets	33,045			$ 36,800
Other assets	51,772				50,084
Total assets	3,849,482				3,633,244
Current liabilities:
Current portion of debt	6,354				6,354
Current portion of acquisition-related liabilities	31,898				31,770
Accounts payable	152,843				109,008
Accrued expenses	118,234				100,029
Current operating lease liabilities	8,609
Intercompany payables	0				0
Billings in excess of costs and estimated earnings	12,476				11,840
Total current liabilities	330,414				259,001
Long-term debt	1,853,414				1,807,502
Acquisition-related liabilities	38,645				45,354
Noncurrent operating lease liabilities	25,329
Other noncurrent liabilities	151,016				135,956
Total liabilities	2,398,818				2,247,813
Total member's interest	1,450,664	$ 1,360,044	$ 1,299,730		1,385,431	$ 1,389,257	$ 1,291,139	$ 1,240,542	$ 1,291,594
Total liabilities and stockholders’ equity/member's interest	3,849,482				3,633,244
Summit Materials, LLC | Eliminations
Current assets:
Cash and cash equivalents	(4,919)				(4,870)
Accounts receivable, net	(194)				(185)
Intercompany receivables	(1,232,088)				(1,125,192)
Cost and estimated earnings in excess of billings	0				0
Inventories	0				0
Other current assets	0				0
Total current assets	(1,237,201)				(1,130,247)
Property, plant and equipment, net	0				0
Goodwill	0				0
Intangible assets, net	0				0
Operating lease right-of-use assets	0
Other assets	(3,609,315)				(3,397,794)
Total assets	(4,846,516)				(4,528,041)
Current liabilities:
Current portion of debt	0				0
Current portion of acquisition-related liabilities	0				0
Accounts payable	(194)				(185)
Accrued expenses	(4,919)				(4,870)
Current operating lease liabilities	0
Intercompany payables	(1,232,088)				(1,125,192)
Billings in excess of costs and estimated earnings	0				0
Total current liabilities	(1,237,201)				(1,130,247)
Long-term debt	0				0
Acquisition-related liabilities	0				0
Noncurrent operating lease liabilities	0
Other noncurrent liabilities	(137,018)				(171,317)
Total liabilities	(1,374,219)				(1,301,564)
Total member's interest	(3,472,297)				(3,226,477)
Total liabilities and stockholders’ equity/member's interest	(4,846,516)				(4,528,041)
Summit Materials, LLC | Issuers | Reportable Legal Entities
Current assets:
Cash and cash equivalents	178,242				117,219
Accounts receivable, net	0				0
Intercompany receivables	449,895				500,765
Cost and estimated earnings in excess of billings	0				0
Inventories	0				0
Other current assets	1,811				1,953
Total current assets	629,948				619,937
Property, plant and equipment, net	11,348				13,300
Goodwill	0				0
Intangible assets, net	0				0
Operating lease right-of-use assets	3,486
Other assets	3,499,351				3,292,851
Total assets	4,144,133				3,926,088
Current liabilities:
Current portion of debt	6,354				6,354
Current portion of acquisition-related liabilities	0				0
Accounts payable	3,318				4,712
Accrued expenses	39,361				45,146
Current operating lease liabilities	750
Intercompany payables	782,697				673,175
Billings in excess of costs and estimated earnings	0				0
Total current liabilities	832,480				729,387
Long-term debt	1,853,414				1,807,502
Acquisition-related liabilities	0				0
Noncurrent operating lease liabilities	3,674
Other noncurrent liabilities	3,901				3,768
Total liabilities	2,693,469				2,540,657
Total member's interest	1,450,664				1,385,431
Total liabilities and stockholders’ equity/member's interest	4,144,133				3,926,088
Summit Materials, LLC | 100% Owned Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	2,197				8,440
Accounts receivable, net	314,249				199,538
Intercompany receivables	782,193				624,427
Cost and estimated earnings in excess of billings	47,033				17,711
Inventories	193,195				210,149
Other current assets	9,382				11,308
Total current assets	1,348,249				1,071,573
Property, plant and equipment, net	1,688,612				1,709,083
Goodwill	1,141,574				1,136,785
Intangible assets, net	24,446				18,460
Operating lease right-of-use assets	24,447
Other assets	160,937				154,080
Total assets	4,388,265				4,089,981
Current liabilities:
Current portion of debt	0				0
Current portion of acquisition-related liabilities	31,898				31,770
Accounts payable	138,284				92,132
Accrued expenses	80,757				57,826
Current operating lease liabilities	6,796
Intercompany payables	429,858				436,564
Billings in excess of costs and estimated earnings	11,543				11,347
Total current liabilities	699,136				629,639
Long-term debt	0				0
Acquisition-related liabilities	38,645				45,354
Noncurrent operating lease liabilities	17,684
Other noncurrent liabilities	205,384				226,137
Total liabilities	960,849				901,130
Total member's interest	3,427,416				3,188,851
Total liabilities and stockholders’ equity/member's interest	4,388,265				4,089,981
Summit Materials, LLC | Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	7,069				7,719
Accounts receivable, net	23,005				15,165
Intercompany receivables	0				0
Cost and estimated earnings in excess of billings	2,682				891
Inventories	3,820				3,702
Other current assets	844				2,800
Total current assets	37,420				30,277
Property, plant and equipment, net	62,347				57,749
Goodwill	57,922				56,243
Intangible assets, net	0				0
Operating lease right-of-use assets	5,112
Other assets	799				947
Total assets	163,600				145,216
Current liabilities:
Current portion of debt	0				0
Current portion of acquisition-related liabilities	0				0
Accounts payable	11,435				12,349
Accrued expenses	3,035				1,927
Current operating lease liabilities	1,063
Intercompany payables	19,533				15,453
Billings in excess of costs and estimated earnings	933				493
Total current liabilities	35,999				30,222
Long-term debt	0				0
Acquisition-related liabilities	0				0
Noncurrent operating lease liabilities	3,971
Other noncurrent liabilities	78,749				77,368
Total liabilities	118,719				107,590
Total member's interest	44,881				37,626
Total liabilities and stockholders’ equity/member's interest	$ 163,600				$ 145,216