| Disaggregation of Revenue [Table Text Block] |
| | | For the Three Months Ended September 30, | | | For the Nine Months Ended September 30, | |
| (In thousands) | | 2026 | | | 2025 | | | 2026 | | | 2025 | |
| Coal royalty revenues | | $ | 35,142 | | | $ | 34,193 | | | $ | 99,509 | | | $ | 101,234 | |
| Production lease minimum revenues | | | 531 | | | | 1,365 | | | | 1,340 | | | | 4,213 | |
| Minimum lease straight-line revenues | | | 4,019 | | | | 4,176 | | | | 12,057 | | | | 12,276 | |
| Oil and gas royalty revenues | | | 1,477 | | | | 1,787 | | | | 5,310 | | | | 6,212 | |
| Carbon neutral revenues | | | 245 | | | | 316 | | | | 524 | | | | 1,201 | |
| Property tax revenues | | | 420 | | | | 2,105 | | | | 3,841 | | | | 5,261 | |
| Wheelage revenues | | | 2,055 | | | | 2,225 | | | | 6,004 | | | | 6,506 | |
| Coal overriding royalty revenues | | | 544 | | | | 297 | | | | 2,970 | | | | 1,633 | |
| Lease amendment revenues | | | 1,117 | | | | 1,699 | | | | 3,559 | | | | 3,010 | |
| Aggregates royalty revenues | | | 1,198 | | | | 1,011 | | | | 3,562 | | | | 2,770 | |
| Other revenues | | | 241 | | | | 441 | | | | 729 | | | | 854 | |
| Royalty and other mineral rights revenues | | $ | 46,989 | | | $ | 49,615 | | | $ | 139,405 | | | $ | 145,170 | |
| Transportation and processing services revenues | | | 1,506 | | | | 1,293 | | | | 8,303 | | | | 7,207 | |
| Total Mineral Rights segment revenues from contracts with customers | | $ | 48,495 | | | $ | 50,908 | | | $ | 147,708 | | | $ | 152,377 | |
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| Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] |
| | | September 30, | | | December 31, | |
| (In thousands) | | 2026 | | | 2025 | |
| Contract assets | | | | | | | | |
| Accounts receivable, net | | $ | 24,258 | | | $ | 24,372 | |
| Other current assets, net | | | — | | | | 84 | |
| Other long-term assets, net | | | 5,461 | | | | 5,281 | |
| | | | | | | | | |
| Contract liabilities | | | | | | | | |
| Accounts payable | | $ | 125 | | | $ | 211 | |
| Current portion of deferred revenue | | | 5,789 | | | | 6,663 | |
| Deferred revenue | | | 59,075 | | | | 58,067 | |
| | | For the Nine Months Ended September 30, | |
| (In thousands) | | 2026 | | | 2025 | |
| Balance at beginning of period (current and non-current) | | $ | 64,730 | | | $ | 60,155 | |
| Increase due to minimums and lease amendment fees | | | 16,808 | | | | 14,479 | |
| Recognition of previously deferred revenue | | | (16,674 | ) | | | (17,115 | ) |
| Balance at end of period (current and non-current) | | $ | 64,864 | | | $ | 57,519 | |
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