Unaudited Consolidated Statements of Changes in Stockholders' Equity / Members' Interest and Redeemable Noncontrolling 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 Lp
Beginning balance at Dec. 29, 2018	$ 1,342,145					$ 129,739		$ 2,681	$ (23,616)	$ 1,117	$ 0	$ 1,194,204	$ 14,404
Beginning balance at Dec. 29, 2018		$ 1,385,431			$ 1,396,241		$ 12,806		(23,616)
Beginning balance (in shares) at Dec. 29, 2018			111,658,927							111,658,927	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		766			766
Net (loss) income	(71,501)	(91,564)				(68,772)	(91,564)						(2,729)
LP Unit exchanges (in shares)										17,500
LP Unit exchanges	0									$ 0		122	(122)
Other comprehensive (loss) income, net of tax	1,650	2,192						1,584	2,192				66
Stock option exercises (in shares)										43,142
Stock option exercises	767									$ 1		766
Distributions		(2,500)			(2,500)
Share-based compensation	5,906	5,906			5,906							5,906
Shares redeemed to settle taxes and other (in shares)										347,962
Shares redeemed to settle taxes and other	(492)	(501)			(501)					$ 3		(495)
Ending balance at Mar. 30, 2019	1,278,475					60,967		4,265		$ 1,121	$ 0	1,200,503	11,619
Ending balance at Mar. 30, 2019		1,299,730			1,399,912		(78,758)		(21,424)
Ending balance (in shares) at Mar. 30, 2019										112,067,531	99
Beginning balance at Dec. 29, 2018	1,342,145					129,739		2,681	(23,616)	$ 1,117	$ 0	1,194,204	14,404
Beginning balance at Dec. 29, 2018		1,385,431			1,396,241		12,806		(23,616)
Beginning balance (in shares) at Dec. 29, 2018			111,658,927							111,658,927	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	24,726	47,136
LP Unit exchanges (in shares)			107,336
Other comprehensive (loss) income, net of tax	2,349	3,115
Shares redeemed to settle taxes and other (in shares)			509,735
Ending balance at Sep. 28, 2019	1,386,778					153,134		4,938	(20,501)	$ 1,124	$ 0	1,212,240	15,342
Ending balance at Sep. 28, 2019		1,450,664			1,411,223		59,942		(20,501)
Ending balance (in shares) at Sep. 28, 2019			112,275,998							112,275,998	99
Beginning balance at Mar. 30, 2019	1,278,475					60,967		4,265		$ 1,121	$ 0	1,200,503	11,619
Beginning balance at Mar. 30, 2019		1,299,730			1,399,912		(78,758)		(21,424)
Beginning balance (in shares) at Mar. 30, 2019										112,067,531	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		18			18
Net (loss) income	37,990	53,501				36,410	53,501						1,580
Other comprehensive (loss) income, net of tax	1,588	2,096						1,526	2,096				62
Stock option exercises (in shares)										1,019
Stock option exercises	17									$ 0		17
Share-based compensation	4,699	4,699			4,699							4,699
Shares redeemed to settle taxes and other (in shares)										4,944
Shares redeemed to settle taxes and other	2									$ 1		2	(1)
Ending balance at Jun. 29, 2019	1,322,771					97,377		5,791		$ 1,122	$ 0	1,205,221	13,260
Ending balance at Jun. 29, 2019		1,360,044			1,404,629		(25,257)		(19,328)
Ending balance (in shares) at Jun. 29, 2019										112,073,494	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		1,775			1,775
Net (loss) income	58,237	85,199				55,757	85,199						2,480
LP Unit exchanges (in shares)										89,836
LP Unit exchanges	0									$ 1		361	(362)
Other comprehensive (loss) income, net of tax	(889)	(1,173)						(853)	(1,173)				(36)
Stock option exercises (in shares)										99,344
Stock option exercises	1,775									$ 1		1,774
Share-based compensation	4,819	4,819			4,819							4,819
Shares redeemed to settle taxes and other (in shares)										13,324
Shares redeemed to settle taxes and other	65											65
Ending balance at Sep. 28, 2019	1,386,778					153,134		4,938	(20,501)	$ 1,124	$ 0	1,212,240	15,342
Ending balance at Sep. 28, 2019		1,450,664			1,411,223		59,942		(20,501)
Ending balance (in shares) at Sep. 28, 2019			112,275,998							112,275,998	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	99						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)
Stock option exercises (in shares)										13,335
Stock option exercises	310									$ 0		310
Distributions		(2,500)			(2,500)
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	99						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)	(3,395)
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	99						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
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
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									$ 0		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	99						114,123,911	99