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Note 5 - Investments (Tables) | 12 Months Ended
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Dec. 31, 2024
Notes Tables |
Investment [Table Text Block] |
​ |  | December 31, |
​ |  | 2024 |  |  | 2023 |
Current investments: |  |  |  |  |  |  |  |
Seeded investment products: |  | ​ |  |  | ​ |
Consolidated VIEs |  | $ | 502.1 |  |  | $ | 382.4 |
Consolidated VREs |  |  | 132.5 |  |  |  | 242.9 |
Unconsolidated VIEs and VREs |  |  | 127.1 |  |  |  | 27.1 |
Separately managed accounts |  |  | 41.9 |  |  |  | 44.1 |
Total seeded investment products |  |  | 803.6 |  |  |  | 696.5 |
Investments related to deferred compensation plans |  |  | 29.8 |  |  |  | 12.0 |
Other investments |  |  | 5.8 |  |  |  | 8.1 |
Total current investments |  | $ | 839.2 |  |  | $ | 716.6 |
Non-current investments: |  |  |  |  |  |  |  |
Equity method investments |  |  | 23.1 |  |  |  | 29.5 |
Total investments |  | $ | 862.3 |  |  | $ | 746.1 |
Gain (Loss) on Securities [Table Text Block] |
​ |  | Year ended December 31, |
​ |  | 2024 |  |  | 2023 |  |  | 2022 |
Seeded investment products and hedges, net |  | $ | 36.4 |  |  | $ | 20.3 |  |  | $ | (15.2 | )
Third-party ownership interests in seeded investment products |  |  | 37.5 |  |  |  | 34.7 |  |  |  | (97.9 | )
Equity method investments |  |  | (5.6 | ) |  |  | (13.5 | ) |  |  | 2.9 |
Other |  |  | 2.5 |  |  |  | 1.9 |  |  |  | (3.1 | )
Investment gains (losses), net |  | $ | 70.8 |  |  | $ | 43.4 |  |  | $ | (113.3 | )
Cash Flows Related to Investment Securities and Other Assets [Table Text Block] |
​ |  | Year ended December 31, |
​ |  | 2024 |  |  | 2023 |  |  | 2022 |
​ |  | Purchases |  |  | Sales, |  |  |  |  |  |  | Purchases |  |  | Sales, |  |  |  |  |  |  | Purchases |  |  | Sales, |  |  |  |  |
​ |  | and |  |  | settlements and |  |  | Net |  |  | and |  |  | settlements and |  |  | Net |  |  | and |  |  | settlements and |  |  | Net |
​ |  | settlements |  |  | maturities |  |  | cash flow |  |  | settlements |  |  | maturities |  |  | cash flow |  |  | settlements |  |  | maturities |  |  | cash flow |
Investments by consolidated seeded investment products |  | $ | (127.8 | ) |  | $ | 26.4 |  |  | $ | (101.4 | ) |  | $ | (227.3 | ) |  | $ | 2.4 |  |  | $ | (224.9 | ) |  | $ | (88.4 | ) |  | $ | 44.5 |  |  | $ | (43.9 | )
Investments |  |  | (211.0 | ) |  |  | 174.0 |  |  |  | (37.0 | ) |  |  | (180.6 | ) |  |  | 120.9 |  |  |  | (59.7 | ) |  |  | (143.1 | ) |  |  | 187.7 |  |  |  | 44.6 |

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