DIVESTITURES - Additional Information (Details) a in Thousands, $ in Millions		3 Months Ended		6 Months Ended
	Aug. 31, 2025 USD ($)	Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2026 USD ($)	Mar. 31, 2025 USD ($)	Oct. 01, 2025 USD ($)	Sep. 30, 2025 USD ($)	Aug. 05, 2025 a
Business Combination [Line Items]
Gain on sale of properties, plants and equipment, net		$ 1.7	$ (0.1)	$ 217.4	$ 2.3
Intercompany Sales | Containerboard Business
Business Combination [Line Items]
Cost of products sold			6.6		14.3
Net sales			$ 6.6		$ 14.3
Disposal Group, Held-for-Sale, Not Discontinued Operations | Fiber Solutions segment, Soterra LLC
Business Combination [Line Items]
Number of acres | a								173
Aggregate carrying value							$ 231.4
Purchase price						$ 462.0
Gain on sale of properties, plants and equipment, net				$ 216.2
Discontinued Operations, Disposed of by Sale | Greif Containerboard Solutions, LLC
Business Combination [Line Items]
Purchase price	$ 1,804.7
Incurred transaction costs	$ 23.4