Revenue Recognition - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Change in Contract with Customer, Asset [Abstract]
Balance—December 28, 2019	$ 13,088
Changes in revenue billed, contract price or cost estimates	(4,535)	$ (5,558)	$ 8,702
Other	113
Balance—January 2, 2021	8,666	13,088
Change in Contract with Customer, Liability [Abstract]
Balance—December 28, 2019	13,864
Changes in revenue billed, contract price or cost estimates	2,616	1,988	(5,052)
Other	19
Balance—January 2, 2021	16,499	13,864
Summit Materials, LLC
Change in Contract with Customer, Asset [Abstract]
Balance—December 28, 2019	13,088
Changes in revenue billed, contract price or cost estimates	(4,535)	(5,558)	8,702
Other	113
Balance—January 2, 2021	8,666	13,088
Change in Contract with Customer, Liability [Abstract]
Balance—December 28, 2019	13,864
Changes in revenue billed, contract price or cost estimates	2,616	1,988	$ (5,052)
Other	19
Balance—January 2, 2021	$ 16,499	$ 13,864