| Mineral Properties Disclosure [Table Text Block] |
| | | September 30, 2026 | | | December 31, 2025 | |
| (In thousands) | | Carrying Value | | | Accumulated Depletion | | | Net Book Value | | | Carrying Value | | | Accumulated Depletion | | | Net Book Value | |
| Coal properties | | $ | 653,808 | | | $ | (325,376 | ) | | $ | 328,432 | | | $ | 653,808 | | | $ | (304,412 | ) | | $ | 349,396 | |
| Aggregates properties | | | 8,655 | | | | (4,597 | ) | | | 4,058 | | | | 8,655 | | | | (4,364 | ) | | | 4,291 | |
| Oil and gas royalty properties | | | 12,354 | | | | (10,695 | ) | | | 1,659 | | | | 12,354 | | | | (10,584 | ) | | | 1,770 | |
| Other | | | 13,141 | | | | (1,611 | ) | | | 11,530 | | | | 13,142 | | | | (1,612 | ) | | | 11,530 | |
| Total mineral rights, net | | $ | 687,958 | | | $ | (342,279 | ) | | $ | 345,679 | | | $ | 687,959 | | | $ | (320,972 | ) | | $ | 366,987 | |
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