Note 4 - Investments - Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Investments at cost	$ 758,317	$ 705,269
Investments at fair value	695,697	647,244
Biotechnology Sector [Member]
Investments at cost	121,373	94,280
Investments at fair value	88,688	61,829
Medical Device Sector [Member]
Investments at cost	254,995	227,893
Investments at fair value	249,545	222,485
Communications Sector [Member]
Investments at cost	522	522
Investments at fair value	3,154	3,142
Consumer-related Sector [Member]
Investments at cost	31,039	31,204
Investments at fair value	21,927	21,846
Data Storage Sector [Member]
Investments at cost	417	417
Investments at fair value	221	230
Internet and Media Sector [Member]
Investments at cost	329	329
Investments at fair value	67	151
Networking Sector [Member]
Investments at cost	364	364
Investments at fair value	0	0
Software Sector [Member]
Investments at cost	197,406	199,608
Investments at fair value	187,173	190,911
Alternative Energy [Member]
Investments at cost	4,919	4,914
Investments at fair value	4,911	4,905
Energy Efficiency [Member]
Investments at cost	125	125
Investments at fair value	23	9
Other Sustainability Sector [Member]
Investments at cost	51,164	50,321
Investments at fair value	50,213	49,315
Diagnostics Sector [Member]
Investments at cost	11,772	11,553
Investments at fair value	11,573	11,369
Other Healthcare Sector [Member]
Investments at cost	38,777	38,668
Investments at fair value	34,599	34,508
Software, Healthcare Information and Service [Member]
Investments at cost	45,115	45,071
Investments at fair value	43,603	46,544
Debt [Member]
Investments at cost	655,048	602,488
Investments at fair value	645,629	596,025
Warrant [Member]
Investments at cost	15,515	14,868
Investments at fair value	16,685	16,648
Other Debt Obligations [Member]
Investments at cost	45,099	45,258
Investments at fair value	3,664	3,444
Equity Securities [Member]
Investments at cost	42,655	42,655
Investments at fair value	$ 29,719	$ 31,127