Incomes Taxes - Deferred Tax Asset Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019
Valuation Allowance:
Beginning balance	$ (1,675)	$ (19,366)
Current year decreases (increases) from operations	0	17,691
Ending balance	(1,675)	(1,675)
Summit Materials, LLC
Valuation Allowance:
Beginning balance	(1,675)	(4,261)
Release of valuation allowance and other	0	2,586
Ending balance	$ (1,675)	$ (1,675)