GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 29, 2019	Mar. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 29, 2019	Jun. 30, 2018
Condensed Consolidating Statements of Operations
Revenue	$ 600,891		$ 600,890		$ 933,530	$ 915,311
Cost of revenue (excluding items shown separately below)	405,493		410,932		672,497	658,793
Depreciation, depletion, amortization and accretion	53,625		49,731		109,013	96,689
Operating income	80,422		77,279		22,751	25,754
Income (loss) from operation before taxes	54,697		49,103		(44,841)	(23,551)
Income tax expense (benefit)	16,707		12,190		(11,330)	(4,516)
Net income (loss) attributable to Summit Inc.	36,410		35,509		(32,362)	(18,220)
Comprehensive (loss) income attributable to member of Summit Materials, LLC	38,002		35,331		(29,186)	(20,961)
Net income (loss)	37,990	$ (71,501)	36,913	$ (55,948)	(33,511)	(19,035)
Summit Materials, LLC
Condensed Consolidating Statements of Operations
Revenue	600,891		600,890		933,530	915,311
Cost of revenue (excluding items shown separately below)	405,493		410,932		672,497	658,793
General and administrative expenses	61,351		62,948		129,269	134,075
Depreciation, depletion, amortization and accretion	53,625		49,731		109,013	96,689
Operating income	80,422		77,279		22,751	25,754
Other (income) loss, net	(3,676)		(767)		8,086	(8,422)
Interest expense (income)	29,283		28,776		59,220	57,346
Income (loss) from operation before taxes	54,815		49,270		(44,555)	(23,170)
Income tax expense (benefit)	1,314		2,668		(6,492)	(1,176)
Net income (loss) attributable to Summit Inc.	53,501		46,602		(38,063)	(21,994)
Comprehensive (loss) income attributable to member of Summit Materials, LLC	55,597		44,918		(33,775)	(25,787)
Net income (loss)	53,501	$ (91,564)	46,602	$ (68,596)	(38,063)	(21,994)
Summit Materials, LLC | Eliminations
Condensed Consolidating Statements of Operations
Revenue	(2,761)		(1,619)		(3,944)	(3,037)
Cost of revenue (excluding items shown separately below)	(2,761)		(1,619)		(3,944)	(3,037)
General and administrative expenses	0		0		0	0
Depreciation, depletion, amortization and accretion	0		0		0	0
Operating income	0		0		0	0
Other (income) loss, net	95,001		89,481		65,497	71,267
Interest expense (income)	0		0		0	0
Income (loss) from operation before taxes	(95,001)		(89,481)		(65,497)	(71,267)
Income tax expense (benefit)	0		0		0	0
Net income (loss) attributable to Summit Inc.	(95,001)		(89,481)		(65,497)	(71,267)
Comprehensive (loss) income attributable to member of Summit Materials, LLC	(92,905)		(91,165)		(61,209)	(75,060)
Issuers | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	0		0		0	0
Cost of revenue (excluding items shown separately below)	0		0		0	0
General and administrative expenses	9,353		13,276		24,284	34,224
Depreciation, depletion, amortization and accretion	992		635		1,945	1,345
Operating income	(10,345)		(13,911)		(26,229)	(35,569)
Other (income) loss, net	(95,320)		(89,659)		(51,710)	(72,294)
Interest expense (income)	31,022		28,946		62,718	58,527
Income (loss) from operation before taxes	53,953		46,802		(37,237)	(21,802)
Income tax expense (benefit)	452		200		826	192
Net income (loss) attributable to Summit Inc.	53,501		46,602		(38,063)	(21,994)
Comprehensive (loss) income attributable to member of Summit Materials, LLC	55,597		44,918		(33,775)	(25,787)
100% Owned Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	578,157		580,157		896,338	880,815
Cost of revenue (excluding items shown separately below)	390,640		396,629		646,807	634,756
General and administrative expenses	49,288		47,287		99,319	94,463
Depreciation, depletion, amortization and accretion	51,167		47,929		104,087	92,922
Operating income	87,062		88,312		46,125	58,674
Other (income) loss, net	(2,985)		(687)		(4,926)	(7,544)
Interest expense (income)	(2,947)		(1,362)		(5,911)	(3,565)
Income (loss) from operation before taxes	92,994		90,361		56,962	69,783
Income tax expense (benefit)	85		2,036		(7,652)	(1,417)
Net income (loss) attributable to Summit Inc.	92,909		88,325		64,614	71,200
Comprehensive (loss) income attributable to member of Summit Materials, LLC	93,046		87,964		64,917	69,844
Non-Guarantors | Summit Materials, LLC | Reportable Legal Entities
Condensed Consolidating Statements of Operations
Revenue	25,495		22,352		41,136	37,533
Cost of revenue (excluding items shown separately below)	17,614		15,922		29,634	27,074
General and administrative expenses	2,710		2,385		5,666	5,388
Depreciation, depletion, amortization and accretion	1,466		1,167		2,981	2,422
Operating income	3,705		2,878		2,855	2,649
Other (income) loss, net	(372)		98		(775)	149
Interest expense (income)	1,208		1,192		2,413	2,384
Income (loss) from operation before taxes	2,869		1,588		1,217	116
Income tax expense (benefit)	777		432		334	49
Net income (loss) attributable to Summit Inc.	2,092		1,156		883	67
Comprehensive (loss) income attributable to member of Summit Materials, LLC	$ (141)		$ 3,201		$ (3,708)	$ 5,216