| Investment Holdings, Schedule of Investments [Table Text Block] |
| | | March 31, 2026 | | | December 31, 2025 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Investments | | | | | | | | | | | | | | | | |
| Debt | | $ | 655,048 | | | $ | 645,629 | | | $ | 602,488 | | | $ | 596,025 | |
| Warrants | | | 15,515 | | | | 16,685 | | | | 14,868 | | | | 16,648 | |
| Other | | | 45,099 | | | | 3,664 | | | | 45,258 | | | | 3,444 | |
| Equity | | | 42,655 | | | | 29,719 | | | | 42,655 | | | | 31,127 | |
| Total investments | | $ | 758,317 | | | $ | 695,697 | | | $ | 705,269 | | | $ | 647,244 | |
| | | March 31, 2026 | | | December 31, 2025 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Life Science | | | | | | | | | | | | | | | | |
| Biotechnology | | $ | 121,373 | | | $ | 88,688 | | | $ | 94,280 | | | $ | 61,829 | |
| Medical Device | | | 254,995 | | | | 249,545 | | | | 227,893 | | | | 222,485 | |
| Technology | | | | | | | | | | | | | | | | |
| Communications | | | 522 | | | | 3,154 | | | | 522 | | | | 3,142 | |
| Consumer-Related | | | 31,039 | | | | 21,927 | | | | 31,204 | | | | 21,846 | |
| Data Storage | | | 417 | | | | 221 | | | | 417 | | | | 230 | |
| Internet and Media | | | 329 | | | | 67 | | | | 329 | | | | 151 | |
| Networking | | | 364 | | | | — | | | | 364 | | | | — | |
| Software | | | 197,406 | | | | 187,173 | | | | 199,608 | | | | 190,911 | |
| Sustainability | | | | | | | | | | | | | | | | |
| Alternative Energy | | | 4,919 | | | | 4,911 | | | | 4,914 | | | | 4,905 | |
| Energy Efficiency | | | 125 | | | | 23 | | | | 125 | | | | 9 | |
| Other Sustainability | | | 51,164 | | | | 50,213 | | | | 50,321 | | | | 49,315 | |
| Healthcare Information and Services | | | | | | | | | | | | | | | | |
| Diagnostics | | | 11,772 | | | | 11,573 | | | | 11,553 | | | | 11,369 | |
| Other | | | 38,777 | | | | 34,599 | | | | 38,668 | | | | 34,508 | |
| Software | | | 45,115 | | | | 43,603 | | | | 45,071 | | | | 46,544 | |
| Total investments | | $ | 758,317 | | | $ | 695,697 | | | $ | 705,269 | | | $ | 647,244 | |
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