Note 4 - Investments - Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Investments at cost	$ 705,269	$ 766,774
Investments at fair value	647,244	697,891
Biotechnology Sector [Member]
Investments at cost	94,280	118,221
Investments at fair value	61,829	97,562
Medical Device Sector [Member]
Investments at cost	227,893	213,564
Investments at fair value	222,485	205,634
Communications Sector [Member]
Investments at cost	522	11,711
Investments at fair value	3,142	14,763
Consumer-related Sector [Member]
Investments at cost	31,204	74,421
Investments at fair value	21,846	36,667
Data Storage Sector [Member]
Investments at cost	417	417
Investments at fair value	230	350
Internet and Media Sector [Member]
Investments at cost	329	329
Investments at fair value	151	999
Networking Sector [Member]
Investments at cost	364	10,070
Investments at fair value	0	9,827
Semiconductor Sector [Member]
Investments at cost	0	45
Investments at fair value	0	0
Software Sector [Member]
Investments at cost	199,608	154,040
Investments at fair value	190,911	151,043
Alternative Energy [Member]
Investments at cost	4,914	0
Investments at fair value	4,905	0
Energy Efficiency [Member]
Investments at cost	125	9,818
Investments at fair value	9	9,712
Other Sustainability Sector [Member]
Investments at cost	50,321	66,151
Investments at fair value	49,315	66,079
Diagnostics Sector [Member]
Investments at cost	11,553	16,704
Investments at fair value	11,369	13,701
Other Healthcare Sector [Member]
Investments at cost	38,668	39,533
Investments at fair value	34,508	40,144
Software, Healthcare Information and Service [Member]
Investments at cost	45,071	51,750
Investments at fair value	46,544	51,410
Debt [Member]
Investments at cost	602,488	678,167
Investments at fair value	596,025	638,779
Warrant [Member]
Investments at cost	14,868	15,956
Investments at fair value	16,648	18,277
Other Debt Obligations [Member]
Investments at cost	45,258	36,709
Investments at fair value	3,444	14,637
Equity Securities [Member]
Investments at cost	42,655	35,942
Investments at fair value	$ 31,127	$ 26,198