Note 6 - Fair Value - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 |
Investments in U.S. Treasury Bills with maturities greater than 3 months | $ 89,155 | $ 186,001 |
Investments in equity securities | 196,583 | 195,585 |
Total investments in securities | 285,738 | 381,586 |
Securities sold, not yet purchased | 5,918 | 2,874 |
US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 89,155 | 186,001 |
Common Stock [Member] | | |
Investments in equity securities | 191,346 | 189,977 |
Mutual Fund [Member] | | |
Investments in equity securities | 1,186 | 906 |
Other Investments [Member] | | |
Investments in equity securities | 4,051 | 4,702 |
Fair Value, Recurring [Member] | | |
Cash equivalents | 315,017 | 216,313 |
Total investments in securities | 285,738 | 381,586 |
Investments in affiliated registered investment companies | 126,751 | 126,210 |
Investment, fair value | 412,489 | 507,796 |
Total assets at fair value | 727,506 | 724,109 |
Securities sold, not yet purchased | 5,918 | 2,874 |
Total liabilities at fair value | 5,918 | 2,874 |
Fair Value, Recurring [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with maturities greater than 3 months | 89,155 | 186,001 |
Fair Value, Recurring [Member] | Common Stock [Member] | | |
Investments in equity securities | 191,346 | 189,977 |
Securities sold, not yet purchased | 5,035 | 2,509 |
Fair Value, Recurring [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 1,186 | 906 |
Investments in affiliated registered investment companies | 73,703 | 69,438 |
Fair Value, Recurring [Member] | Other Investments [Member] | | |
Investments in equity securities | 4,051 | 4,702 |
Securities sold, not yet purchased | 883 | 365 |
Fair Value, Recurring [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 44,692 | 56,772 |
Fair Value, Recurring [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 8,356 | |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Cash equivalents | 315,017 | 216,313 |
Total investments in securities | 281,651 | 376,991 |
Investments in affiliated registered investment companies | 118,395 | 114,724 |
Investment, fair value | 400,046 | 491,715 |
Total assets at fair value | 715,063 | 708,028 |
Securities sold, not yet purchased | 5,614 | 2,641 |
Total liabilities at fair value | 5,614 | 2,641 |
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 | 89,155 | 186,001 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | | |
Investments in equity securities | 187,963 | 185,952 |
Securities sold, not yet purchased | 5,035 | 2,509 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member] | | |
Investments in equity securities | 1,186 | 906 |
Investments in affiliated registered investment companies | 73,703 | 69,438 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Investments [Member] | | |
Investments in equity securities | 3,347 | 4,132 |
Securities sold, not yet purchased | 579 | 132 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 44,692 | 45,286 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 0 | |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 1,833 | 2,307 |
Investments in affiliated registered investment companies | 0 | 0 |
Investment, fair value | 1,833 | 2,307 |
Total assets at fair value | 1,833 | 2,307 |
Securities sold, not yet purchased | 304 | 233 |
Total liabilities at fair value | 304 | 233 |
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 | 1,348 | 1,990 |
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 | 485 | 317 |
Securities sold, not yet purchased | 304 | 233 |
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 | |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 2,254 | 2,288 |
Investments in affiliated registered investment companies | 8,356 | 11,486 |
Investment, fair value | 10,610 | 13,774 |
Total assets at fair value | 10,610 | 13,774 |
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 | 219 | 253 |
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 | $ 11,486 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Securities From Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | $ 8,356 | |