INCOME TAXES - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 28, 2020	Sep. 28, 2019	Mar. 30, 2019	Dec. 28, 2019
Income Taxes [Line Items]
Income tax benefit	$ (22,901,000)		$ (28,037,000)
Valuation allowance	1,675,000			$ 1,675,000
Income tax provision, interest or penalties recognized	0		0
Deferred tax asset, Investment in limited partnership	1,200,000
Tax receivable agreement liability	327,957,000			326,965,000
Tax Receivable Agreement
Income Taxes [Line Items]
Tax receivable agreement liability	328,000,000.0			$ 327,000,000.0
Summit Holdings LP
Income Taxes [Line Items]
Distributions to LP Unitholders	$ 0		0
Summit Holdings LP | Tax Receivable Agreement
Income Taxes [Line Items]
Percentage of benefits to be paid on tax receivable agreement	85.00%
Summit Materials, LLC
Income Taxes [Line Items]
Income tax benefit	$ (5,884,000)		(7,806,000)
Income tax provision, interest or penalties recognized		$ 0	$ 0
Class A Common Stock
Income Taxes [Line Items]
Exchanges during period (in shares)	196,542		17,500