INCOME TAXES - Schedule of Components of Earnings Before Income Taxes, Provision for Income Taxes and Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Income Tax Disclosure [Abstract]
U.S. loss	$ (127.3)	$ (226.2)	$ (22.7)
Foreign income	73.6	38.0	50.3
(Loss) income before income taxes	(53.7)	(188.2)	27.6
Computed tax at the U.S. federal statutory rate of 21%	(11.3)	(39.5)	5.8
Foreign income rate differential, mining, and withholding taxes, net of U.S. federal deduction	12.7	11.3	9.6
Benefit recognized on Canadian law change	0.0	0.0	(6.2)
Percentage depletion in excess of basis	(3.0)	(2.4)	(2.7)
Non-deductible compensation	1.2	2.0	3.1
Other domestic tax reserves, net of reversals	0.2	0.2	(2.6)
State income taxes, net of federal income tax benefit	(5.0)	(8.4)	(1.3)
Change in valuation allowance on deferred tax asset	33.0	46.9	11.1
Global Intangible Low-Taxed Income and Base Erosion and Anti-Abuse Tax	0.0	0.0	1.1
Goodwill impairment	0.0	8.5	0.0
Tax on repatriated amounts	0.0	0.0	(0.7)
Share based payment awards	1.3	0.0	0.0
Other, net	(3.0)	(0.7)	(0.1)
Total provision for income taxes	$ 26.1	$ 17.9	$ 17.1
Effective tax rate	(49.00%)	(10.00%)	62.00%