PROPERTY, PLANT AND EQUIPMENT - Other Plant (Details) - USD ($) $ in Thousands |
Sep. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Property, plant and equipment | ||
| Other plant assets | $ 804,945 | $ 853,410 |
| Less allowances for depreciation, amortization and depletion | (725,089) | (760,477) |
| Net other plant | 79,856 | 92,933 |
| Mine assets | ||
| Property, plant and equipment | ||
| Less allowances for depreciation, amortization and depletion | (197,058) | (232,575) |
| Net other plant | 56,484 | 71,348 |
| Mine assets | Colowyo Mine | ||
| Property, plant and equipment | ||
| Other plant assets | 247,255 | 297,636 |
| Mine assets | New Horizon Mine | ||
| Property, plant and equipment | ||
| Other plant assets | 6,287 | 6,287 |
| Non-utility assets | ||
| Property, plant and equipment | ||
| Other plant assets | 551,402 | 549,487 |
| Less allowances for depreciation, amortization and depletion | (528,030) | (527,902) |
| Net other plant | $ 23,372 | $ 21,585 |
| Elk Ridge | ||
| Property, Plant, and Equipment [Line Items] | ||
| Ownership interest held by parent (as a percent) | 100.00% |