Note 5 - Fair Value - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023 |
Investments in U.S. Treasury Bills with maturities greater than 3 months | $ 115,829 | $ 89,155 |
Investments in equity securities | 249,669 | 196,583 |
Total investments in securities | 365,498 | 285,738 |
Securities sold, not yet purchased | 7,376 | 5,918 |
US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 115,829 | 89,155 |
Common Stock [Member] | | |
Investments in equity securities | 244,874 | 191,346 |
Mutual Fund [Member] | | |
Investments in equity securities | 1,450 | 1,186 |
Other Investments [Member] | | |
Investments in equity securities | 3,345 | 4,051 |
Fair Value, Recurring [Member] | | |
Cash equivalents | 259,247 | 315,017 |
Total investments in securities | 365,498 | 285,738 |
Investments in affiliated registered investment companies | 140,355 | 126,751 |
Investment, fair value | 505,853 | 412,489 |
Total assets at fair value | 765,100 | 727,506 |
Securities sold, not yet purchased | 7,376 | 5,918 |
Total liabilities at fair value | 7,376 | 5,918 |
Fair Value, Recurring [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 115,829 | 89,155 |
Fair Value, Recurring [Member] | Common Stock [Member] | | |
Investments in equity securities | 244,874 | 191,346 |
Securities sold, not yet purchased | 6,838 | 5,035 |
Fair Value, Recurring [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 1,450 | 1,186 |
Investments in affiliated registered investment companies | 82,457 | 73,703 |
Fair Value, Recurring [Member] | Other Investments [Member] | | |
Investments in equity securities | 3,345 | 4,051 |
Securities sold, not yet purchased | 538 | 883 |
Fair Value, Recurring [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 48,042 | 44,692 |
Fair Value, Recurring [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 9,856 | 8,356 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Cash equivalents | 259,247 | 315,017 |
Total investments in securities | 361,867 | 281,651 |
Investments in affiliated registered investment companies | 130,499 | 118,395 |
Investment, fair value | 492,366 | 400,046 |
Total assets at fair value | 751,613 | 715,063 |
Securities sold, not yet purchased | 6,873 | 5,614 |
Total liabilities at fair value | 6,873 | 5,614 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 115,829 | 89,155 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | | |
Investments in equity securities | 242,017 | 187,963 |
Securities sold, not yet purchased | 6,838 | 5,035 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 1,450 | 1,186 |
Investments in affiliated registered investment companies | 82,457 | 73,703 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Investments [Member] | | |
Investments in equity securities | 2,571 | 3,347 |
Securities sold, not yet purchased | 35 | 579 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 48,042 | 44,692 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 1,470 | 1,833 |
Investments in affiliated registered investment companies | 0 | 0 |
Investment, fair value | 1,470 | 1,833 |
Total assets at fair value | 1,470 | 1,833 |
Securities sold, not yet purchased | 503 | 304 |
Total liabilities at fair value | 503 | 304 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stock [Member] | | |
Investments in equity securities | 822 | 1,348 |
Securities sold, not yet purchased | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 0 | 0 |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Investments [Member] | | |
Investments in equity securities | 648 | 485 |
Securities sold, not yet purchased | 503 | 304 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 2,161 | 2,254 |
Investments in affiliated registered investment companies | 9,856 | 8,356 |
Investment, fair value | 12,017 | 10,610 |
Total assets at fair value | 12,017 | 10,610 |
Securities sold, not yet purchased | 0 | 0 |
Total liabilities at fair value | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stock [Member] | | |
Investments in equity securities | 2,035 | 2,035 |
Securities sold, not yet purchased | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 0 | 0 |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Investments [Member] | | |
Investments in equity securities | 126 | 219 |
Securities sold, not yet purchased | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | $ 9,856 | $ 8,356 |