PROPERTY, PLANT AND EQUIPMENT, NET - Schedule of Property, Plant and Equipment, Net (Details) - USD ($) $ in Millions |
Jun. 30, 2026 |
Sep. 30, 2025 |
|---|---|---|
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | $ 1,946.3 | $ 1,953.6 |
| Less: accumulated depreciation, depletion, and amortization | (1,199.2) | (1,183.5) |
| Property, plant and equipment, net | 747.1 | 770.1 |
| Land, buildings and structures, and leasehold improvements | ||
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | 539.7 | 554.1 |
| Machinery and equipment | ||
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | 1,156.6 | 1,154.9 |
| Office furniture and equipment | ||
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | 13.5 | 23.9 |
| Mineral interests | ||
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | 168.0 | 169.1 |
| Construction in progress | ||
| Property, Plant, and Equipment [Line Items] | ||
| Property, plant and equipment, gross | $ 68.5 | $ 51.6 |