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DEBT - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 28, 2019	Mar. 30, 2019	Mar. 15, 2019	Dec. 29, 2018	Jun. 01, 2017	Jul. 08, 2015
Debt Instrument [Line Items]
Total debt	$ 1,873,745			$ 1,828,159
Current portion of long-term debt	6,354			6,354
Long-term debt	1,867,391			1,821,805
Gross amount	1,875,844
Debt discount	2,099
Term Loan, due 2024
Debt Instrument [Line Items]
Total debt	624,672			629,268
Gross amount	625,800			630,600
Debt discount	1,200			1,300
8 1/2% Senior Notes, due 2022
Debt Instrument [Line Items]
Total debt	0			$ 250,000
Debt instrument interest rate (as a percent)				8.50%
6 1/8% Senior Notes, due 2023
Debt Instrument [Line Items]
Total debt	649,073			$ 648,891
Gross amount	650,000			650,000
Debt discount	$ 1,000			1,100
Debt instrument interest rate (as a percent)	6.125%
5 1/8% Senior Notes, due 2025
Debt Instrument [Line Items]
Total debt	$ 300,000			300,000
Debt instrument interest rate (as a percent)	5.125%
Summit Materials, LLC
Debt Instrument [Line Items]
Total debt	$ 1,873,745			1,828,159
Current portion of long-term debt	6,354			6,354
Long-term debt	1,867,391			1,821,805
Gross amount	1,875,844
Debt discount	2,099
Summit Materials, LLC \| Term Loan, due 2024
Debt Instrument [Line Items]
Total debt	624,672			629,268
Gross amount	625,800			630,600
Debt discount	1,200			1,300
Summit Materials, LLC \| 8 1/2% Senior Notes, due 2022
Debt Instrument [Line Items]
Total debt	0			$ 250,000
Debt instrument interest rate (as a percent)		8.50%		8.50%
Summit Materials, LLC \| 6 1/8% Senior Notes, due 2023
Debt Instrument [Line Items]
Total debt	649,073			$ 648,891
Gross amount	650,000			650,000
Debt discount	$ 1,000			1,100
Debt instrument interest rate (as a percent)	6.125%					6.125%
Summit Materials, LLC \| 5 1/8% Senior Notes, due 2025
Debt Instrument [Line Items]
Total debt	$ 300,000			300,000	$ 300,000
Debt instrument interest rate (as a percent)	5.125%				5.125%
Senior Notes \| 6 1/2% Senior Notes, due 2027
Debt Instrument [Line Items]
Total debt	$ 300,000			0
Debt instrument interest rate (as a percent)	6.50%
Senior Notes \| Summit Materials, LLC \| 6 1/2% Senior Notes, due 2027
Debt Instrument [Line Items]
Total debt	$ 300,000		$ 300,000	$ 0
Debt instrument interest rate (as a percent)	6.50%		6.50%