STOCKHOLDERS' EQUITY/MEMBERS' INTEREST - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 28, 2020	Mar. 30, 2019	Dec. 28, 2019
Changes in each component of accumulated other comprehensive loss
Beginning balance	$ 1,444,773	$ 1,342,145	$ 1,342,145
Postretirement liability adjustment, net of tax	(6,093)	1,704
(Loss) income on cash flow hedges, net of tax		(120)
Ending balance	1,395,859	1,278,475	1,444,773
Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Postretirement liability adjustment, net of tax		2,358	(8,359)
(Loss) income on cash flow hedges, net of tax		(166)
Change in retirement plans
Changes in each component of accumulated other comprehensive loss
Beginning balance	2,171	3,573	3,573
Ending balance	2,171	3,573	2,171
Change in retirement plans \| Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Beginning balance	(6,317)	(4,392)	(4,392)
Ending balance	(6,317)	(4,392)	(6,317)
Foreign currency translation adjustments
Changes in each component of accumulated other comprehensive loss
Beginning balance	1,277	(2,147)	(2,147)
Postretirement liability adjustment, net of tax	(6,093)	1,704
Ending balance	(4,816)	(443)	1,277
Foreign currency translation adjustments \| Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Beginning balance	(14,654)	(19,370)	(19,370)
Postretirement liability adjustment, net of tax		2,358	(8,359)
Ending balance	(23,013)	(17,012)	(14,654)
Cash flow hedge adjustments
Changes in each component of accumulated other comprehensive loss
Beginning balance	0	1,255	1,255
(Loss) income on cash flow hedges, net of tax		(120)
Ending balance	0	1,135	0
Cash flow hedge adjustments \| Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Beginning balance	0	146	146
(Loss) income on cash flow hedges, net of tax		(166)
Ending balance	0	(20)	0
Accumulated other comprehensive income (loss)
Changes in each component of accumulated other comprehensive loss
Beginning balance	3,448	2,681	2,681
Ending balance	(2,645)	4,265	3,448
Accumulated other comprehensive income (loss) \| Summit Materials, LLC
Changes in each component of accumulated other comprehensive loss
Beginning balance	(20,971)	(23,616)	(23,616)
Ending balance	$ (29,330)	$ (21,424)	$ (20,971)

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature, attributable to parent entity.

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Amount, after tax and reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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