GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Condensed Consolidating Statements of Operations
Revenue	$ 367,190	$ 332,639
Cost of revenue (excluding items shown separately below)	286,155	267,004
Depreciation, depletion, amortization and accretion	51,778	55,388
Operating loss	(41,720)	(57,671)
Loss from operations before taxes	(69,627)	(99,538)
Income tax expense (benefit)	(22,901)	(28,037)
Net loss attributable to Summit Inc.	(44,979)	(68,772)
Comprehensive loss attributable to member of Summit Materials, LLC	(51,072)	(67,188)
Summit Materials, LLC
Condensed Consolidating Statements of Operations
Revenue	367,190	332,639
Cost of revenue (excluding items shown separately below)	286,155	267,004
General and administrative expenses	70,977	67,918
Depreciation, depletion, amortization and accretion	51,778	55,388
Operating loss	(41,720)	(57,671)
Other (income) loss, net	89	11,762
Interest expense (income)	27,700	29,937
Loss from operations before taxes	(69,509)	(99,370)
Income tax expense (benefit)	(5,884)	(7,806)
Net loss attributable to Summit Inc.	(63,625)	(91,564)
Comprehensive loss attributable to member of Summit Materials, LLC	(71,984)	(89,372)
Summit Materials, LLC | Eliminations
Condensed Consolidating Statements of Operations
Revenue	(3,333)	(1,183)
Cost of revenue (excluding items shown separately below)	(3,333)	(1,183)
General and administrative expenses	0	0
Depreciation, depletion, amortization and accretion	0	0
Operating loss	0	0
Other (income) loss, net	(16,068)	(29,504)
Interest expense (income)	0	0
Loss from operations before taxes	16,068	29,504
Income tax expense (benefit)	0	0
Net loss attributable to Summit Inc.	16,068	29,504
Comprehensive loss attributable to member of Summit Materials, LLC	7,709	31,696
Issuers | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	0	0
Cost of revenue (excluding items shown separately below)	0	0
General and administrative expenses	13,830	14,930
Depreciation, depletion, amortization and accretion	989	952
Operating loss	(14,819)	(15,882)
Other (income) loss, net	15,292	43,611
Interest expense (income)	33,214	31,697
Loss from operations before taxes	(63,325)	(91,190)
Income tax expense (benefit)	300	374
Net loss attributable to Summit Inc.	(63,625)	(91,564)
Comprehensive loss attributable to member of Summit Materials, LLC	(71,984)	(89,372)
100% Owned Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	354,330	318,181
Cost of revenue (excluding items shown separately below)	278,195	256,166
General and administrative expenses	54,123	50,032
Depreciation, depletion, amortization and accretion	49,473	52,920
Operating loss	(27,461)	(40,937)
Other (income) loss, net	(306)	(1,942)
Interest expense (income)	(6,714)	(2,964)
Loss from operations before taxes	(20,441)	(36,031)
Income tax expense (benefit)	(5,717)	(7,737)
Net loss attributable to Summit Inc.	(14,724)	(28,294)
Comprehensive loss attributable to member of Summit Materials, LLC	(14,724)	(28,128)
Non-Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	16,193	15,641
Cost of revenue (excluding items shown separately below)	11,293	12,021
General and administrative expenses	3,024	2,956
Depreciation, depletion, amortization and accretion	1,316	1,516
Operating loss	560	(852)
Other (income) loss, net	1,171	(403)
Interest expense (income)	1,200	1,204
Loss from operations before taxes	(1,811)	(1,653)
Income tax expense (benefit)	(467)	(443)
Net loss attributable to Summit Inc.	(1,344)	(1,210)
Comprehensive loss attributable to member of Summit Materials, LLC	$ 7,015	$ (3,568)