Income Taxes - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 29, 2018	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Income Taxes [Line Items]
Deferred tax assets subject to TRA		$ 378,500,000
Income tax expense (benefit)		(12,185,000)	$ 17,101,000	$ 59,747,000
Reduction of net deferred tax assets due to change in statutory rate	$ 17,600,000
Unrecognized tax benefits that if recognized would affect the annual effective tax rate	22,700,000	0.0	41,500,000	22,700,000
Unrecognized tax benefits	22,663,000	0	41,548,000	22,663,000
Deferred tax asset, valuation allowance	19,366,000	1,675,000	1,675,000	19,366,000
Deferred tax asset, Investment in limited partnership		2,400,000	1,100,000
Tax receivable agreement (benefit) expense		(7,559,000)	16,237,000	(22,684,000)
Tax receivable agreement liability		321,680,000	326,965,000
Penalties and interest expense		0	0	0
Summit Holdings LP
Income Taxes [Line Items]
Distributions to LP Unitholders		0	100,000
Summit Materials, LLC
Income Taxes [Line Items]
Income tax expense (benefit)		1,063,000	6,385,000	10,273,000
Reduction of net deferred tax assets due to change in statutory rate				2,600,000
Unrecognized tax benefits	6,487,000	0	11,619,000	6,487,000
Net operating loss carryforwards		120,300,000
Deferred tax asset, valuation allowance	$ 4,261,000	1,675,000	1,675,000	$ 4,261,000
Penalties and interest expense		$ 0	0
Tax Receivable Agreement
Income Taxes [Line Items]
Tax receivable agreement liability			$ 327,000,000.0
Tax Receivable Agreement | Summit Holdings LP
Income Taxes [Line Items]
Percentage of benefits to be paid on tax receivable agreement		85.00%
Federal
Income Taxes [Line Items]
Net operating loss carryforwards		$ 917,000,000
Federal | Tax Receivable Agreement
Income Taxes [Line Items]
Net operating loss carryforwards		$ 497,000,000