Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance—December 29, 2018	$ 22,663	$ 0
Additions based on tax position in 2019	18,885	22,663
Balance—December 28, 2019	41,548	22,663
Summit Materials, LLC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance—December 29, 2018	6,487	0
Additions based on tax position in 2019	5,132	6,487
Balance—December 28, 2019	$ 11,619	$ 6,487