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Note 4 - Investments - Investments (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Investments at cost | $ 759,182 | $ 721,248
Investments at fair value | 709,085 | 720,026
Biotechnology Sector [Member] |  |
Investments at cost | 145,544 | 193,372
Investments at fair value | 117,781 | 195,006
Medical Device Sector [Member] |  |
Investments at cost | 147,064 | 132,803
Investments at fair value | 145,019 | 135,960
Communications Sector [Member] |  |
Investments at cost | 19,192 | 22,892
Investments at fair value | 22,188 | 26,176
Consumer-related Sector [Member] |  |
Investments at cost | 105,669 | 121,961
Investments at fair value | 101,327 | 114,050
Data Storage Sector [Member] |  |
Investments at cost | 417 | 476
Investments at fair value | 418 | 1,316
Internet and Media Sector [Member] |  |
Investments at cost | 329 | 329
Investments at fair value | 1,582 | 1,159
Networking Sector [Member] |  |
Investments at cost | 5,451 | 11,831
Investments at fair value | 5,297 | 11,710
Power Management Sector [Member] |  |
Investments at cost | 1,585 | 1,585
Investments at fair value | 2,700 | 2,610
Semiconductor Sector [Member] |  |
Investments at cost | 45 | 56
Investments at fair value | 0 | 0
Software Sector [Member] |  |
Investments at cost | 164,133 | 120,157
Investments at fair value | 160,749 | 118,716
Energy Efficiency [Member] |  |
Investments at cost | 111 | 8
Investments at fair value | 13 | 27
Other Sustainability Sector [Member] |  |
Investments at cost | 98,906 | 84,633
Investments at fair value | 80,370 | 85,524
Diagnostics Sector [Member] |  |
Investments at cost | 19,735 | 9,851
Investments at fair value | 19,701 | 9,858
Other Healthcare Sector [Member] |  |
Investments at cost | 102 | 7,559
Investments at fair value | 1,366 | 3,870
Software, Healthcare Information and Service [Member] |  |
Investments at cost | 50,899 | 13,735
Investments at fair value | 50,574 | 14,044
Debt [Member] |  |
Investments at cost | 721,231 | 701,074
Investments at fair value | 670,172 | 686,458
Warrant [Member] |  |
Investments at cost | 16,526 | 14,790
Investments at fair value | 24,594 | 29,712
Other Debt Obligations [Member] |  |
Investments at cost | 6,982 | 1,200
Investments at fair value | 6,430 | 1,300
Equity Securities [Member] |  |
Investments at cost | 14,443 | 4,184
Investments at fair value | $ 7,889 | $ 2,556

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