Note 5 - Fair Value - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022 |
Investments in U.S. Treasury Bills with greater than 3 month maturities | $ 2,961 | $ 186,001 |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 213,345 | 195,585 |
Total investments in securities | 216,306 | 381,586 |
Securities sold, not yet purchased | 5,090 | 2,874 |
US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with greater than 3 month maturities | 2,961 | 186,001 |
Common Stock [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 208,154 | 189,977 |
Mutual Fund [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 1,055 | 906 |
Other Investments [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 4,136 | 4,702 |
Fair Value, Recurring [Member] | | |
Cash equivalents | 378,004 | 216,313 |
Total investments in securities | 216,306 | 381,586 |
Investments in affiliated registered investment companies | 125,292 | 126,210 |
Investment, fair value | 341,598 | 507,796 |
Total assets at fair value | 719,602 | 724,109 |
Securities sold, not yet purchased | 5,090 | 2,874 |
Total liabilities at fair value | 5,090 | 2,874 |
Fair Value, Recurring [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with greater than 3 month maturities | 2,961 | 186,001 |
Fair Value, Recurring [Member] | Common Stock [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 208,154 | 189,977 |
Securities sold, not yet purchased | 4,796 | 2,509 |
Fair Value, Recurring [Member] | Mutual Fund [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 1,055 | 906 |
Investments in affiliated registered investment companies | 70,734 | 69,438 |
Fair Value, Recurring [Member] | Other Investments [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 4,136 | 4,702 |
Securities sold, not yet purchased | 294 | 365 |
Fair Value, Recurring [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 54,558 | 56,772 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Cash equivalents | 378,004 | 216,313 |
Total investments in securities | 212,155 | 376,991 |
Investments in affiliated registered investment companies | 113,936 | 114,724 |
Investment, fair value | 326,091 | 491,715 |
Total assets at fair value | 704,095 | 708,028 |
Securities sold, not yet purchased | 4,854 | 2,641 |
Total liabilities at fair value | 4,854 | 2,641 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with greater than 3 month maturities | 2,961 | 186,001 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 204,871 | 185,952 |
Securities sold, not yet purchased | 4,796 | 2,509 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Mutual Fund [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 1,055 | 906 |
Investments in affiliated registered investment companies | 70,734 | 69,438 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Investments [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 3,268 | 4,132 |
Securities sold, not yet purchased | 58 | 132 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Closed End Funds [Member] | | |
Investments in affiliated registered investment companies | 43,202 | 45,286 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 1,897 | 2,307 |
Investments in affiliated registered investment companies | 0 | 0 |
Investment, fair value | 1,897 | 2,307 |
Total assets at fair value | 1,897 | 2,307 |
Securities sold, not yet purchased | 236 | 233 |
Total liabilities at fair value | 236 | 233 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Bill Securities [Member] | | |
Investments in U.S. Treasury Bills with greater than 3 month maturities | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stock [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 1,248 | 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 (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 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 (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 649 | 317 |
Securities sold, not yet purchased | 236 | 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 3 [Member] | | |
Cash equivalents | 0 | 0 |
Total investments in securities | 2,254 | 2,288 |
Investments in affiliated registered investment companies | 11,356 | 11,486 |
Investment, fair value | 13,610 | 13,774 |
Total assets at fair value | 13,610 | 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 greater than 3 month maturities | 0 | 0 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stock [Member] | | |
Investments in equity securities (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 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 (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 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 (includes GAMCO stock with a fair value of $48.0 million and $36.7 million, respectively) | 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 | $ 11,356 | $ 11,486 |