Incomes Taxes - Deferred Tax Asset Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018
Valuation Allowance:
Beginning balance	$ (19,366)	$ (1,675)
Additional basis from exchanged LP Units	0	(99)
Current year decreases (increases) from operations	17,691	(17,592)
Ending balance	(1,675)	(19,366)
Summit Materials, LLC
Valuation Allowance:
Beginning balance	(4,261)	(1,675)
Current year decreases (increases) from operations	2,586	(2,586)
Ending balance	$ (1,675)	$ (4,261)