| Investment Holdings, Schedule of Investments [Table Text Block] |
| | | December 31, 2025 | | | December 31, 2024 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Investments | | | | | | | | | | | | | | | | |
| Debt | | $ | 602,488 | | | $ | 596,025 | | | $ | 678,167 | | | $ | 638,779 | |
| Warrants | | | 14,868 | | | | 16,648 | | | | 15,956 | | | | 18,277 | |
| Other | | | 45,258 | | | | 3,444 | | | | 36,709 | | | | 14,637 | |
| Equity | | | 42,655 | | | | 31,127 | | | | 35,942 | | | | 26,198 | |
| Total investments | | $ | 705,269 | | | $ | 647,244 | | | $ | 766,774 | | | $ | 697,891 | |
| | | December 31, 2025 | | | December 31, 2024 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Life Science | | | | | | | | | | | | | | | | |
| Biotechnology | | $ | 94,280 | | | $ | 61,829 | | | $ | 118,221 | | | $ | 97,562 | |
| Medical Device | | | 227,893 | | | | 222,485 | | | | 213,564 | | | | 205,634 | |
| Technology | | | | | | | | | | | | | | | | |
| Communications | | | 522 | | | | 3,142 | | | | 11,711 | | | | 14,763 | |
| Consumer-Related | | | 31,204 | | | | 21,846 | | | | 74,421 | | | | 36,667 | |
| Data Storage | | | 417 | | | | 230 | | | | 417 | | | | 350 | |
| Internet and Media | | | 329 | | | | 151 | | | | 329 | | | | 999 | |
| Networking | | | 364 | | | | — | | | | 10,070 | | | | 9,827 | |
| Semiconductors | | | — | | | | — | | | | 45 | | | | — | |
| Software | | | 199,608 | | | | 190,911 | | | | 154,040 | | | | 151,043 | |
| Sustainability | | | | | | | | | | | | | | | | |
| Alternative Energy | | | 4,914 | | | | 4,905 | | | | — | | | | — | |
| Energy Efficiency | | | 125 | | | | 9 | | | | 9,818 | | | | 9,712 | |
| Other Sustainability | | | 50,321 | | | | 49,315 | | | | 66,151 | | | | 66,079 | |
| Healthcare Information and Services | | | | | | | | | | | | | | | | |
| Diagnostics | | | 11,553 | | | | 11,369 | | | | 16,704 | | | | 13,701 | |
| Other | | | 38,668 | | | | 34,508 | | | | 39,533 | | | | 40,144 | |
| Software | | | 45,071 | | | | 46,544 | | | | 51,750 | | | | 51,410 | |
| Total investments | | $ | 705,269 | | | $ | 647,244 | | | $ | 766,774 | | | $ | 697,891 | |
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