Income Taxes - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017	Sep. 30, 2017
Income Taxes [Line Items]
Unrecognized tax benefits			$ 18,885,000	$ 22,663,000	$ 0
Deferred tax assets subject to TRA			384,700,000
Income tax expense (benefit)			17,101,000	59,747,000	(283,977,000)
Reduction of net deferred tax assets due to change in statutory rate	$ 17,600,000	$ 216,900,000
Unrecognized tax benefits	22,663,000	0	41,548,000	22,663,000	0
Unrecognized tax benefits that if recognized would affect the annual effective tax rate	22,700,000		41,500,000	22,700,000
Alternative minimum tax credit			200,000
Deferred tax asset, valuation allowance	19,366,000	1,675,000	1,675,000	19,366,000	1,675,000
Deferred tax asset, Investment in limited partnership	2,000,000.0		1,100,000	2,000,000.0
Tax receivable agreement expense (benefit)			16,237,000	(22,684,000)	271,016,000
Tax receivable agreement liability	309,674,000		326,965,000	309,674,000
Penalties and interest expense			0	0	0
Liability for uncertainty in income taxes		0			0
Summit Holdings LP
Income Taxes [Line Items]
Distributions to LP Unitholders			100,000	100,000
Summit Materials, LLC
Income Taxes [Line Items]
Unrecognized tax benefits			5,038,000	6,487,000	0
Income tax expense (benefit)			6,385,000	10,273,000	(20,345,000)
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	0
Net operating loss carryforwards			120,300,000
Deferred tax asset, valuation allowance	4,261,000	$ 1,675,000	$ 1,675,000	4,261,000	$ 1,675,000
Penalties and interest expense				0
Tax Receivable Agreement
Income Taxes [Line Items]
Tax benefit deemed probable						$ 501,800,000
Tax Receivable Agreement | Summit Holdings LP
Income Taxes [Line Items]
Percentage of benefits to be paid on tax receivable agreement			85.00%		85.00%
Other noncurrent liabilities | Tax Receivable Agreement
Income Taxes [Line Items]
Tax receivable agreement liability	309,700,000			309,700,000
Accrued expenses. | Tax Receivable Agreement
Income Taxes [Line Items]
Tax receivable agreement liability	$ 700,000		$ 0.0	$ 700,000
Federal
Income Taxes [Line Items]
Net operating loss carryforwards			822,000,000
Federal | Tax Receivable Agreement
Income Taxes [Line Items]
Net operating loss carryforwards			$ 411,000,000