Unaudited Consolidated Statements of Changes in Stockholders' Equity / Members' Interest - USD ($) $ in Thousands	Total	Summit Materials, LLC	Common Class A	Common Class B	Members' equity Summit Materials, LLC	Accumulated Earnings (Deficit)	Accumulated Earnings (Deficit) Summit Materials, LLC	Accumulated other comprehensive income (loss)	Accumulated other comprehensive income (loss) Summit Materials, LLC	Common Stock Common Class A	Common Stock Common Class B	Additional Paid-in Capital	Noncontrolling Interest In Summit Holdings
Beginning balance at Dec. 28, 2019	$ 1,444,773					$ 188,805		$ 3,448	$ (20,971)	$ 1,134	$ 0	$ 1,234,020	$ 17,366
Beginning balance at Dec. 28, 2019		$ 1,513,150			$ 1,432,718		$ 101,403		(20,971)
Beginning balance (in shares) at Dec. 28, 2019			113,309,385							113,309,385	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		310			310
Net (loss) income	(46,726)	(63,625)				(44,979)	(63,625)						(1,747)
LP Unit exchanges (in shares)										196,542
LP Unit exchanges	0									$ 2		1,132	(1,134)
Other comprehensive (loss) income, net of tax	(6,313)	(8,359)						(6,093)	(8,359)				(220)
Distributions		(2,500)			(2,500)
Stock option exercises (in shares)										13,335
Stock option exercises	310									$ 0		310
Share-based compensation	4,905	4,905			4,905							4,905
Shares redeemed to settle taxes and other (in shares)										591,335
Shares redeemed to settle taxes and other	(1,090)	(908)			(908)					$ 6		(1,096)
Ending balance at Mar. 28, 2020	1,395,859					143,826		(2,645)		$ 1,142	$ 0	1,239,271	14,265
Ending balance at Mar. 28, 2020		1,442,973			1,434,525		37,778		(29,330)
Ending balance (in shares) at Mar. 28, 2020										114,110,597	99
Beginning balance at Dec. 28, 2019	1,444,773					188,805		3,448	(20,971)	$ 1,134	$ 0	1,234,020	17,366
Beginning balance at Dec. 28, 2019		1,513,150			1,432,718		101,403		(20,971)
Beginning balance (in shares) at Dec. 28, 2019			113,309,385							113,309,385	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	104,930	84,716
LP Unit exchanges (in shares)			196,542
Other comprehensive (loss) income, net of tax	(2,564)
Stock option exercises (in shares)			14,404
Shares redeemed to settle taxes and other (in shares)			603,580
Ending balance at Sep. 26, 2020	1,569,496					291,620		975	(24,366)	$ 1,142	$ 0	1,257,506	18,253
Ending balance at Sep. 26, 2020		1,614,511			1,452,758		186,119		(24,366)
Ending balance (in shares) at Sep. 26, 2020			114,123,911							114,123,911	99
Beginning balance at Mar. 28, 2020	1,395,859					143,826		(2,645)		$ 1,142	$ 0	1,239,271	14,265
Beginning balance at Mar. 28, 2020		1,442,973			1,434,525		37,778		(29,330)
Beginning balance (in shares) at Mar. 28, 2020										114,110,597	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	58,887	70,017				57,064	70,017						1,823
Other comprehensive (loss) income, net of tax	2,225	2,946						2,148	2,946				77
Share-based compensation	4,892	4,892			4,892							4,892
Shares redeemed to settle taxes and other (in shares)										1,351
Shares redeemed to settle taxes and other	0									$ 0		0	0
Ending balance at Jun. 27, 2020	1,461,863					200,890		(497)		$ 1,142	$ 0	1,244,163	16,165
Ending balance at Jun. 27, 2020		1,520,828			1,439,417		107,795		(26,384)
Ending balance (in shares) at Jun. 27, 2020										114,111,948	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		19			19
Net (loss) income	92,769	78,324				90,730	78,324						2,039
Other comprehensive (loss) income, net of tax	1,524	2,018						1,472	2,018				52
Stock option exercises (in shares)										1,069
Stock option exercises	18									$ 0		18
Share-based compensation	13,322	13,322			13,322							13,322
Shares redeemed to settle taxes and other (in shares)										10,894
Shares redeemed to settle taxes and other	0											3	(3)
Ending balance at Sep. 26, 2020	1,569,496					291,620		975	(24,366)	$ 1,142	$ 0	1,257,506	18,253
Ending balance at Sep. 26, 2020		1,614,511			1,452,758		186,119		(24,366)
Ending balance (in shares) at Sep. 26, 2020			114,123,911							114,123,911	99
Beginning balance at Jan. 02, 2021	1,616,268					326,772		5,203	(18,583)	$ 1,145	$ 0	1,264,681	18,467
Beginning balance at Jan. 02, 2021		1,662,768			1,459,211		222,140		(18,583)
Beginning balance (in shares) at Jan. 02, 2021			114,390,595	99						114,390,595	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		15,920			15,920
Net (loss) income	(23,245)	(27,790)				(22,517)	(27,790)						(728)
LP Unit exchanges (in shares)										711,794
LP Unit exchanges	0									$ 7		4,744	(4,751)
Other comprehensive (loss) income, net of tax	1,680	2,126						1,635	2,126				45
Distributions		(2,500)			(2,500)
Stock option exercises (in shares)										863,338
Stock option exercises	15,920									$ 9		15,911
Share-based compensation	5,363	5,363			5,363							5,363
Shares redeemed to settle taxes and other (in shares)										678,605
Shares redeemed to settle taxes and other	(1,426)	(416)			(416)					$ 6		(1,432)
Ending balance at Apr. 03, 2021	1,614,560					304,255		6,838		$ 1,167	$ 0	1,289,267	13,033
Ending balance at Apr. 03, 2021		1,655,471			1,477,578		194,350		(16,457)
Ending balance (in shares) at Apr. 03, 2021										116,644,332	99
Beginning balance at Jan. 02, 2021	1,616,268					326,772		5,203	(18,583)	$ 1,145	$ 0	1,264,681	18,467
Beginning balance at Jan. 02, 2021		1,662,768			1,459,211		222,140		(18,583)
Beginning balance (in shares) at Jan. 02, 2021			114,390,595	99						114,390,595	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	109,891	130,900
LP Unit exchanges (in shares)			1,440,983
Other comprehensive (loss) income, net of tax	506
Stock option exercises (in shares)			1,744,256
Shares redeemed to settle taxes and other (in shares)			989,116
Ending balance at Oct. 02, 2021	1,771,764					435,118		5,670	(17,901)	$ 1,186	$ 0	1,319,897	9,893
Ending balance at Oct. 02, 2021		1,838,190			1,503,051		353,040		(17,901)
Ending balance (in shares) at Oct. 02, 2021			118,564,950	99						118,564,950	99
Beginning balance at Apr. 03, 2021	1,614,560					304,255		6,838		$ 1,167	$ 0	1,289,267	13,033
Beginning balance at Apr. 03, 2021		1,655,471			1,477,578		194,350		(16,457)
Beginning balance (in shares) at Apr. 03, 2021										116,644,332	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		15,846			15,846
Net (loss) income	57,758	69,815				56,659	69,815						1,099
LP Unit exchanges (in shares)										445,540
LP Unit exchanges	0									$ 4		3,631	(3,635)
Other comprehensive (loss) income, net of tax	2,079	2,632						2,028	2,632				51
Stock option exercises (in shares)										847,480
Stock option exercises	15,845									$ 8		15,837
Share-based compensation	4,827	4,827			4,827							4,827
Shares redeemed to settle taxes and other (in shares)										18,536
Shares redeemed to settle taxes and other	(147)									$ 1		(148)
Ending balance at Jul. 03, 2021	1,694,922					360,914		8,866		$ 1,180	$ 0	1,313,414	10,548
Ending balance at Jul. 03, 2021		1,748,591			1,498,251		264,165		(13,825)
Ending balance (in shares) at Jul. 03, 2021										117,955,888	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		650			650
Net (loss) income	75,378	88,875				74,204	88,875						1,174
LP Unit exchanges (in shares)										283,649
LP Unit exchanges	0									$ 3		1,769	(1,772)
Other comprehensive (loss) income, net of tax	(3,253)	(4,076)						(3,196)	(4,076)				(57)
Stock option exercises (in shares)										33,438
Stock option exercises	651									$ 0		651
Share-based compensation	4,685	4,685			4,685							4,685
Shares redeemed to settle taxes and other (in shares)										291,975
Shares redeemed to settle taxes and other	(619)	(535)			(535)					$ 3		(622)
Ending balance at Oct. 02, 2021	$ 1,771,764					$ 435,118		$ 5,670	(17,901)	$ 1,186	$ 0	$ 1,319,897	$ 9,893
Ending balance at Oct. 02, 2021		$ 1,838,190			$ 1,503,051		$ 353,040		$ (17,901)
Ending balance (in shares) at Oct. 02, 2021			118,564,950	99						118,564,950	99