Income Taxes - Schedule of Income Tax Benefit (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Line Items]
Income tax expense (benefit) at federal statutory tax rate	$ 27,100	$ 16,427	$ 20,177
Less: Income tax benefit at federal statutory tax rate for LLC entities	(593)	(658)	(561)
State and local income taxes	5,067	3,792	4,894
Permanent differences	(3,345)	(6,272)	(5,537)
Effective tax rate change	4,257	(2,006)	4,034
Unrecognized tax benefits	(41,548)	18,885	22,663
Tax receivable agreement (benefit) expense	(6)	2,436	(8,282)
Change in valuation allowance	0	(17,691)	17,592
Other	(3,117)	2,188	4,767
Income tax expense (benefit)	(12,185)	17,101	59,747
Summit Materials, LLC
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Line Items]
Income tax expense (benefit) at federal statutory tax rate	25,577	19,947	15,563
Less: Income tax benefit at federal statutory tax rate for LLC entities	(17,647)	(15,387)	(13,863)
State and local income taxes	2,073	1,680	1,614
Permanent differences	2,479	13	(1,194)
Effective tax rate change	681	(725)	(1,148)
Unrecognized tax benefits	(11,525)	5,038	6,487
Change in valuation allowance	0	(2,478)	2,586
Other	(575)	(1,703)	228
Income tax expense (benefit)	$ 1,063	$ 6,385	$ 10,273