| Investment Holdings, Schedule of Investments [Table Text Block] |
| | | June 30, 2026 | | | December 31, 2025 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Investments | | | | | | | | | | | | | | | | |
| Debt | | $ | 682,425 | | | $ | 647,999 | | | $ | 602,488 | | | $ | 596,025 | |
| Warrants | | | 15,155 | | | | 16,703 | | | | 14,868 | | | | 16,648 | |
| Other | | | 37,962 | | | | 4,037 | | | | 45,258 | | | | 3,444 | |
| Equity | | | 42,110 | | | | 7,757 | | | | 42,655 | | | | 31,127 | |
| Equity interest in RoHo Joint Venture | | | 215 | | | | 215 | | | | — | | | | — | |
| Total investments | | $ | 777,867 | | | $ | 676,711 | | | $ | 705,269 | | | $ | 647,244 | |
| | | June 30, 2026 | | | December 31, 2025 | |
| | | Cost | | | Fair Value | | | Cost | | | Fair Value | |
| | | (In thousands) | |
| Life Science | | | | | | | | | | | | | | | | |
| Biotechnology | | $ | 117,812 | | | $ | 86,231 | | | $ | 94,280 | | | $ | 61,829 | |
| Medical Device | | | 230,456 | | | | 232,913 | | | | 227,893 | | | | 222,485 | |
| Technology | | | | | | | | | | | | | | | | |
| Communications | | | 6,286 | | | | 7,459 | | | | 522 | | | | 3,142 | |
| Consumer-Related | | | 30,840 | | | | 19,800 | | | | 31,204 | | | | 21,846 | |
| Data Storage | | | 417 | | | | 197 | | | | 417 | | | | 230 | |
| Internet and Media | | | 329 | | | | 78 | | | | 329 | | | | 151 | |
| Networking | | | 364 | | | | — | | | | 364 | | | | — | |
| Software | | | 237,700 | | | | 228,691 | | | | 199,608 | | | | 190,911 | |
| Sustainability | | | | | | | | | | | | | | | | |
| Alternative Energy | | | — | | | | — | | | | 4,914 | | | | 4,905 | |
| Energy Efficiency | | | 125 | | | | 15 | | | | 125 | | | | 9 | |
| Other Sustainability | | | 57,406 | | | | 18,223 | | | | 50,321 | | | | 49,315 | |
| Healthcare Information and Services | | | | | | | | | | | | | | | | |
| Diagnostics | | | 12,001 | | | | 10,560 | | | | 11,553 | | | | 11,369 | |
| Other | | | 38,889 | | | | 30,041 | | | | 38,668 | | | | 34,508 | |
| Software | | | 45,027 | | | | 42,288 | | | | 45,071 | | | | 46,544 | |
| Investment funds | | | | | | | | | | | | | | | | |
| RoHo Capital Opportunity Fund LLC | | | 215 | | | | 215 | | | | — | | | | — | |
| Total investments | | $ | 777,867 | | | $ | 676,711 | | | $ | 705,269 | | | $ | 647,244 | |
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