Unaudited Consolidated Statements of Changes in Stockholders' Equity / Members' Interest and Redeemable Noncontrolling Interest - USD ($) $ in Thousands	Total	Summit Materials, LLC	Class A Common Stock	Class B Common Stock	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 Class A Common Stock	Common Stock Class B Common Stock	Additional Paid-in Capital	Noncontrolling Interest in Summit Holdings
Beginning balance at Dec. 30, 2017	$ 1,271,721					$ 95,833		$ 7,386		$ 1,104	$ 0	$ 1,154,220	$ 13,178
Beginning balance at Dec. 30, 2017		$ 1,291,594			$ 1,359,760		$ (51,031)		$ (17,135)
Beginning balance (in shares) at Dec. 30, 2017			110,350,594							110,350,594	100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		15,475			15,475
Net income	(55,948)	(68,596)				(53,729)	(68,596)						(2,219)
LP Unit exchanges (in shares)										104,104
LP Unit exchanges	0									$ 1		485	(486)
Other comprehensive income, net of tax	(2,629)	(2,109)						(2,563)	(2,109)				(66)
Stock option exercises (in shares)										856,915
Stock option exercises	15,477									$ 9		15,468
Distributions		(2,509)			(2,509)
Share-based compensation	8,507	8,507			8,507							8,507
Distributions from partnership	(9)												(9)
Shares redeemed to settle taxes and other (in shares)										176,960
Shares redeemed to settle taxes and other	(1,772)	(1,820)			(1,820)					$ 2		(1,774)
Ending balance at Mar. 31, 2018	1,235,347					42,104		4,823		$ 1,116	$ 0	1,176,906	10,398
Ending balance at Mar. 31, 2018		1,240,542			1,379,413		(119,627)		(19,244)
Ending balance (in shares) at Mar. 31, 2018										111,488,573	100
Beginning balance at Dec. 30, 2017	1,271,721					95,833		7,386		$ 1,104	$ 0	1,154,220	13,178
Beginning balance at Dec. 30, 2017		1,291,594			1,359,760		(51,031)		(17,135)
Beginning balance (in shares) at Dec. 30, 2017			110,350,594							110,350,594	100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	54,957	68,433
LP Unit exchanges (in shares)			254,102
Other comprehensive income, net of tax	$ (1,308)	(1,736)
Shares redeemed to settle taxes and other (in shares)	1,049,856		1,049,856
Ending balance at Sep. 29, 2018	$ 1,358,870					148,902		6,134		$ 1,117	$ 0	1,188,707	14,010
Ending balance at Sep. 29, 2018		1,389,257			1,390,726		17,402		(18,871)
Ending balance (in shares) at Sep. 29, 2018			111,654,552							111,654,552	99
Beginning balance at Mar. 31, 2018	1,235,347					42,104		4,823		$ 1,116	$ 0	1,176,906	10,398
Beginning balance at Mar. 31, 2018		1,240,542			1,379,413		(119,627)		(19,244)
Beginning balance (in shares) at Mar. 31, 2018										111,488,573	100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		140			140
Net income	36,913	46,602				35,509	46,602						1,404
LP Unit exchanges (in shares)										125,554
LP Unit exchanges	0									$ 1		365	(366)
Other comprehensive income, net of tax	(229)	(1,684)						(178)	(1,684)				(51)
Stock option exercises	139											139
Distributions		(60)			(60)
Share-based compensation	5,683	5,683			5,683							5,683
Distributions from partnership	(60)												(60)
Shares redeemed to settle taxes and other (in shares)										15,111
Shares redeemed to settle taxes and other	(22)	(84)			(84)							(22)
Ending balance at Jun. 30, 2018	1,277,771					77,613		4,645		$ 1,117	$ 0	1,183,071	11,325
Ending balance at Jun. 30, 2018		1,291,139			1,385,092		(73,025)		(20,928)
Ending balance (in shares) at Jun. 30, 2018										111,629,238	100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	73,992	90,427				71,289	90,427						2,703
LP Unit exchanges (in shares)										24,444
LP Unit exchanges	0											79	(79)
Other comprehensive income, net of tax	1,550	2,057						1,489	2,057				61
Stock option exercises (in shares)										6,983
Share-based compensation	5,643	5,643			5,643							5,643
Shares redeemed to settle taxes and other (in shares)										(6,113)	(1)
Shares redeemed to settle taxes and other	(86)	(9)			(9)							(86)
Ending balance at Sep. 29, 2018	1,358,870					148,902		6,134		$ 1,117	$ 0	1,188,707	14,010
Ending balance at Sep. 29, 2018		1,389,257			1,390,726		17,402		(18,871)
Ending balance (in shares) at Sep. 29, 2018			111,654,552							111,654,552	99
Beginning balance at Dec. 29, 2018	1,342,145					129,739		2,681		$ 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	99						111,658,927	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net contributed capital		766			766
Net income	(71,501)	(91,564)				(68,772)	(91,564)						(2,729)
LP Unit exchanges (in shares)										17,500
LP Unit exchanges	0											122	(122)
Other comprehensive 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		$ 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	99						111,658,927	99
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24,726	47,136
LP Unit exchanges (in shares)			107,336
Other comprehensive income, net of tax	$ 2,349	3,115
Shares redeemed to settle taxes and other (in shares)	509,735		509,735
Ending balance at Sep. 28, 2019	$ 1,386,778					153,134		4,938		$ 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	99						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 income	37,990	53,501				36,410	53,501						1,580
Other comprehensive income, net of tax	1,588	2,096						1,526	2,096				62
Stock option exercises (in shares)										1,019
Stock option exercises	17											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 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 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		$ 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	99						112,275,998	99