Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Current assets:
Cash and cash equivalents	$ 418,181	$ 311,319
Accounts receivable, net	254,696	253,256
Costs and estimated earnings in excess of billings	8,666	13,088
Inventories	200,308	204,787
Other current assets	11,428	13,831
Total current assets	893,279	796,281
Property, plant and equipment including finance lease right-of-use assets, net	1,850,169	1,747,449
Goodwill	1,201,291	1,199,699	$ 1,192,028
Intangible assets, net	47,852	23,498
Operating lease right-of-use assets	28,543	32,777
Other assets	55,000	55,519
Total assets	4,308,011	4,067,556	3,857,641
Current liabilities:
Current portion of debt	6,354	7,942
Current portion of acquisition-related liabilities	10,265	32,700
Accounts payable	120,813	116,359
Accrued expenses	160,570	120,005
Current operating lease liabilities	8,188	8,427
Billings in excess of costs and estimated earnings	16,499	13,864
Total current liabilities	322,689	299,297
Long-term debt	1,892,347	1,851,057
Acquisition-related liabilities	12,246	19,801
Noncurrent operating lease liabilities	21,500	25,381
Other noncurrent liabilities	121,281	100,282
Total liabilities	2,691,743	2,622,783
Total liabilities and stockholders' equity / member's interest	4,308,011	4,067,556
Summit Materials, LLC
Current assets:
Cash and cash equivalents	418,181	311,319
Accounts receivable, net	254,696	253,256
Intercompany receivables	0	0
Costs and estimated earnings in excess of billings	8,666	13,088
Inventories	200,308	204,787
Other current assets	11,428	13,831
Total current assets	893,279	796,281
Property, plant and equipment including finance lease right-of-use assets, net	1,850,169	1,747,449
Goodwill	1,202,291	1,200,699	1,193,028
Intangible assets, net	47,852	23,498
Operating lease right-of-use assets	28,543	32,777
Other assets	55,000	55,519
Total assets	4,077,134	3,856,223	3,633,244
Current liabilities:
Current portion of debt	6,354	7,942
Current portion of acquisition-related liabilities	7,827	30,200
Accounts payable	121,422	116,970
Accrued expenses	160,801	120,237
Current operating lease liabilities	8,188	8,427
Intercompany payables	0	0
Billings in excess of costs and estimated earnings	16,499	13,864
Total current liabilities	321,091	297,640
Long-term debt	1,892,347	1,851,057
Acquisition-related liabilities	12,246	17,666
Noncurrent operating lease liabilities	21,500	25,381
Other noncurrent liabilities	167,182	151,329
Total liabilities	2,414,366	2,343,073
Total member's interest	1,662,768	1,513,150	$ 1,385,431	$ 1,291,594
Total liabilities and stockholders' equity / member's interest	4,077,134	3,856,223
Summit Materials, LLC | Eliminations
Current assets:
Cash and cash equivalents	(3,641)	(6,477)
Accounts receivable, net	109	(33)
Intercompany receivables	(1,707,752)	(1,385,708)
Costs and estimated earnings in excess of billings	0	0
Inventories	0	0
Other current assets	0	0
Total current assets	(1,711,284)	(1,392,218)
Property, plant and equipment including finance lease right-of-use assets, net	0	0
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	0	0
Other assets	(4,175,921)	(3,709,690)
Total assets	(5,887,205)	(5,101,908)
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	0	0
Accounts payable	109	(33)
Accrued expenses	(3,641)	(6,477)
Current operating lease liabilities	0	0
Intercompany payables	(1,707,752)	(1,385,708)
Billings in excess of costs and estimated earnings	0	0
Total current liabilities	(1,711,284)	(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	(1,875,705)	(1,529,236)
Total member's interest	(4,011,500)	(3,572,672)
Total liabilities and stockholders' equity / member's interest	(5,887,205)	(5,101,908)
Issuers | Summit Materials, LLC | Reportable Legal Entities
Current assets:
Cash and cash equivalents	401,074	302,474
Accounts receivable, net	4	0
Intercompany receivables	404,459	443,323
Costs and estimated earnings in excess of billings	0	0
Inventories	0	0
Other current assets	2,840	1,763
Total current assets	808,377	747,560
Property, plant and equipment including finance lease right-of-use assets, net	9,410	11,602
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	2,615	3,316
Other assets	4,022,729	3,596,161
Total assets	4,843,131	4,358,639
Current liabilities:
Current portion of debt	6,354	7,942
Current portion of acquisition-related liabilities	0	0
Accounts payable	3,889	4,588
Accrued expenses	54,108	51,043
Current operating lease liabilities	913	764
Intercompany payables	1,215,043	922,356
Billings in excess of costs and estimated earnings	0	0
Total current liabilities	1,280,307	986,693
Long-term debt	1,892,347	1,851,057
Acquisition-related liabilities	0	0
Noncurrent operating lease liabilities	2,567	3,480
Other noncurrent liabilities	5,142	4,259
Total liabilities	3,180,363	2,845,489
Total member's interest	1,662,768	1,513,150
Total liabilities and stockholders' equity / member's interest	4,843,131	4,358,639
100% Owned Guarantors | Summit Materials, LLC | Reportable Legal Entities
Current assets:
Cash and cash equivalents	10,287	5,488
Accounts receivable, net	230,199	234,053
Intercompany receivables	1,303,293	942,385
Costs and estimated earnings in excess of billings	7,504	12,291
Inventories	193,417	199,794
Other current assets	6,797	10,308
Total current assets	1,751,497	1,404,319
Property, plant and equipment including finance lease right-of-use assets, net	1,746,045	1,674,443
Goodwill	1,142,083	1,142,063
Intangible assets, net	47,852	23,498
Operating lease right-of-use assets	21,880	24,551
Other assets	207,699	168,314
Total assets	4,917,056	4,437,188
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	7,827	30,200
Accounts payable	108,805	103,812
Accrued expenses	106,320	72,970
Current operating lease liabilities	6,114	6,571
Intercompany payables	485,401	447,827
Billings in excess of costs and estimated earnings	15,508	12,183
Total current liabilities	729,975	673,563
Long-term debt	0	0
Acquisition-related liabilities	12,246	17,666
Noncurrent operating lease liabilities	16,062	18,047
Other noncurrent liabilities	208,540	203,919
Total liabilities	966,823	913,195
Total member's interest	3,950,233	3,523,993
Total liabilities and stockholders' equity / member's interest	4,917,056	4,437,188
Non-Guarantors | Summit Materials, LLC | Reportable Legal Entities
Current assets:
Cash and cash equivalents	10,461	9,834
Accounts receivable, net	24,384	19,236
Intercompany receivables	0	0
Costs and estimated earnings in excess of billings	1,162	797
Inventories	6,891	4,993
Other current assets	1,791	1,760
Total current assets	44,689	36,620
Property, plant and equipment including finance lease right-of-use assets, net	94,714	61,404
Goodwill	60,208	58,636
Intangible assets, net	0	0
Operating lease right-of-use assets	4,048	4,910
Other assets	493	734
Total assets	204,152	162,304
Current liabilities:
Current portion of debt	0	0
Current portion of acquisition-related liabilities	0	0
Accounts payable	8,619	8,603
Accrued expenses	4,014	2,701
Current operating lease liabilities	1,161	1,092
Intercompany payables	7,308	15,525
Billings in excess of costs and estimated earnings	991	1,681
Total current liabilities	22,093	29,602
Long-term debt	0	0
Acquisition-related liabilities	0	0
Noncurrent operating lease liabilities	2,871	3,854
Other noncurrent liabilities	117,921	80,169
Total liabilities	142,885	113,625
Total member's interest	61,267	48,679
Total liabilities and stockholders' equity / member's interest	$ 204,152	$ 162,304