GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 28, 2020	Dec. 28, 2019	Mar. 30, 2019	Dec. 29, 2018
Current assets:
Cash and cash equivalents	$ 199,114	$ 311,319
Accounts receivable, net	231,215	253,256
Cost and estimated earnings in excess of billings	11,319	13,088
Inventories	231,413	204,787
Other current assets	15,195	13,831
Total current assets	688,256	796,281
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	1,768,703	1,747,449
Goodwill	1,195,557	1,199,699
Intangible assets, net	22,793	23,498
Operating lease right-of-use assets	30,527	32,777
Other assets	48,510	55,519
Total assets	3,992,268	4,067,556
Current liabilities:
Current portion of debt	7,942	7,942
Current portion of acquisition-related liabilities	30,923	32,700
Accounts payable	122,350	116,359
Accrued expenses	102,460	120,005
Current operating lease liabilities	8,131	8,427
Billings in excess of costs and estimated earnings	14,022	13,864
Total current liabilities	285,828	299,297
Long-term debt	1,850,289	1,851,057
Acquisition-related liabilities	13,024	19,801
Noncurrent operating lease liabilities	23,280	25,381
Other noncurrent liabilities	96,031	100,282
Total liabilities	2,596,409	2,622,783
Total liabilities and stockholders’ equity/member's interest	3,992,268	4,067,556
Summit Materials, LLC
Current assets:
Cash and cash equivalents	199,114	311,319
Accounts receivable, net	231,215	253,256
Intercompany receivables	0	0
Cost and estimated earnings in excess of billings	11,319	13,088
Inventories	231,413	204,787
Other current assets	15,195	13,831
Total current assets	688,256	796,281
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	1,768,703	1,747,449
Goodwill	1,196,557	1,200,699
Intangible assets, net	22,793	23,498
Operating lease right-of-use assets	30,527	32,777
Other assets	48,510	55,519
Total assets	3,755,346	3,856,223
Current liabilities:
Current portion of debt	7,942	7,942
Current portion of acquisition-related liabilities	28,671	30,200
Accounts payable	122,961	116,970
Accrued expenses	102,691	120,237
Current operating lease liabilities	8,131	8,427
Intercompany payables	0	0
Billings in excess of costs and estimated earnings	14,022	13,864
Total current liabilities	284,418	297,640
Long-term debt	1,850,289	1,851,057
Acquisition-related liabilities	13,024	17,666
Noncurrent operating lease liabilities	23,280	25,381
Other noncurrent liabilities	141,362	151,329
Total liabilities	2,312,373	2,343,073
Total member's interest	1,442,973	1,513,150	$ 1,299,730	$ 1,385,431
Total liabilities and stockholders’ equity/member's interest	3,755,346	3,856,223
Summit Materials, LLC | Eliminations
Current assets:
Cash and cash equivalents	(3,687)	(6,477)
Accounts receivable, net	(177)	(33)
Intercompany receivables	(1,308,726)	(1,385,708)
Cost and estimated earnings in excess of billings	0	0
Inventories	0	0
Other current assets	0	0
Total current assets	(1,312,590)	(1,392,218)
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	0	0
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	0	0
Other assets	(3,675,562)	(3,709,690)
Total assets	(4,988,152)	(5,101,908)
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	0	0
Accounts payable	(177)	(33)
Accrued expenses	(3,687)	(6,477)
Current operating lease liabilities	0	0
Intercompany payables	(1,308,726)	(1,385,708)
Billings in excess of costs and estimated earnings	0	0
Total current liabilities	(1,312,590)	(1,392,218)
Long-term debt	0	0
Acquisition-related liabilities	0	0
Noncurrent operating lease liabilities	0	0
Other noncurrent liabilities	(137,020)	(137,018)
Total liabilities	(1,449,610)	(1,529,236)
Total member's interest	(3,538,542)	(3,572,672)
Total liabilities and stockholders’ equity/member's interest	(4,988,152)	(5,101,908)
Summit Materials, LLC | Issuers | Reportable Legal Entities
Current assets:
Cash and cash equivalents	190,794	302,474
Accounts receivable, net	0	0
Intercompany receivables	414,855	443,323
Cost and estimated earnings in excess of billings	0	0
Inventories	0	0
Other current assets	3,042	1,763
Total current assets	608,691	747,560
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	11,067	11,602
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	3,144	3,316
Other assets	3,570,946	3,596,161
Total assets	4,193,848	4,358,639
Current liabilities:
Current portion of debt	7,942	7,942
Current portion of acquisition-related liabilities	0	0
Accounts payable	3,949	4,588
Accrued expenses	35,401	51,043
Current operating lease liabilities	800	764
Intercompany payables	845,943	922,356
Billings in excess of costs and estimated earnings	0	0
Total current liabilities	894,035	986,693
Long-term debt	1,850,289	1,851,057
Acquisition-related liabilities	0	0
Noncurrent operating lease liabilities	3,258	3,480
Other noncurrent liabilities	3,293	4,259
Total liabilities	2,750,875	2,845,489
Total member's interest	1,442,973	1,513,150
Total liabilities and stockholders’ equity/member's interest	4,193,848	4,358,639
Summit Materials, LLC | 100% Owned Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	1,645	5,488
Accounts receivable, net	216,747	234,053
Intercompany receivables	893,871	942,385
Cost and estimated earnings in excess of billings	10,522	12,291
Inventories	226,500	199,794
Other current assets	10,506	10,308
Total current assets	1,359,791	1,404,319
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	1,700,780	1,674,443
Goodwill	1,142,063	1,142,063
Intangible assets, net	22,793	23,498
Operating lease right-of-use assets	23,071	24,551
Other assets	152,451	168,314
Total assets	4,400,949	4,437,188
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	28,671	30,200
Accounts payable	112,763	103,812
Accrued expenses	68,750	72,970
Current operating lease liabilities	6,358	6,571
Intercompany payables	445,880	447,827
Billings in excess of costs and estimated earnings	12,984	12,183
Total current liabilities	675,406	673,563
Long-term debt	0	0
Acquisition-related liabilities	13,024	17,666
Noncurrent operating lease liabilities	16,854	18,047
Other noncurrent liabilities	196,100	203,919
Total liabilities	901,384	913,195
Total member's interest	3,499,565	3,523,993
Total liabilities and stockholders’ equity/member's interest	4,400,949	4,437,188
Summit Materials, LLC | Non-Guarantors | Reportable Legal Entities
Current assets:
Cash and cash equivalents	10,362	9,834
Accounts receivable, net	14,645	19,236
Intercompany receivables	0	0
Cost and estimated earnings in excess of billings	797	797
Inventories	4,913	4,993
Other current assets	1,647	1,760
Total current assets	32,364	36,620
Property, Plant, And Equipment And Finance Lease Right-of-Use Asset, After Accumulated Depreciation And Amortization	56,856	61,404
Goodwill	54,494	58,636
Intangible assets, net	0	0
Operating lease right-of-use assets	4,312	4,910
Other assets	675	734
Total assets	148,701	162,304
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	0	0
Accounts payable	6,426	8,603
Accrued expenses	2,227	2,701
Current operating lease liabilities	973	1,092
Intercompany payables	16,903	15,525
Billings in excess of costs and estimated earnings	1,038	1,681
Total current liabilities	27,567	29,602
Long-term debt	0	0
Acquisition-related liabilities	0	0
Noncurrent operating lease liabilities	3,168	3,854
Other noncurrent liabilities	78,989	80,169
Total liabilities	109,724	113,625
Total member's interest	38,977	48,679
Total liabilities and stockholders’ equity/member's interest	$ 148,701	$ 162,304