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. 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	(71,501)	(91,564)				(68,772)	(91,564)						(2,729)
LP Unit exchanges (in shares)			17,500							17,500
LP Unit exchanges	0									$ 0		122	(122)
Other comprehensive (income) loss, 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)			391,104							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	(21,424)	$ 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							112,067,531	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	(46,726)	(63,625)				(44,979)	(63,625)						(1,747)
LP Unit exchanges (in shares)			196,542							196,542
LP Unit exchanges	0									$ 2		1,132	(1,134)
Other comprehensive (income) loss, net of tax	(6,313)	(8,359)						(6,093)	(8,359)				(220)
Stock option exercises (in shares)			13,335							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							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)	(29,330)	$ 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						114,110,597	99