Note 5 - Derivative Instruments - Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Futures [Member]
Notional value of derivative	$ 832.0	$ 789.0
Futures [Member] \| Seeded Investment Products [Member]
Notional value of derivative	192.1	160.5
Foreign Currency, Forward Contracts and Swaps [Member]
Notional value of derivative	316.2	328.2
Foreign Currency, Forward Contracts and Swaps [Member] \| Not Designated as Hedging Instrument, Economic Hedge [Member]
Notional value of derivative	41.8	38.4
Foreign Currency, Forward Contracts and Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	234.4	196.6
Credit Default Swaps [Member]
Notional value of derivative	135.8	148.5
Credit Default Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	3.0	4.3
Total Return Swaps [Member]
Notional value of derivative	97.2	69.7
Total Return Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	5.0	10.3
Interest Rate Swap [Member] \| Seeded Investment Products [Member]
Notional value of derivative	$ 14.0	$ 13.9