Note 5 - Derivative Instruments - Derivative Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Futures [Member]
Notional value of derivative	$ 1,564.1	$ 789.0
Futures [Member] \| Seeded Investment Products [Member]
Notional value of derivative	634.5	160.5
Foreign Currency, Forward Contracts and Swaps [Member]
Notional value of derivative	295.4	328.2
Foreign Currency, Forward Contracts and Swaps [Member] \| Not Designated as Hedging Instrument, Economic Hedge [Member]
Notional value of derivative	43.0	38.4
Foreign Currency, Forward Contracts and Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	264.7	196.6
Credit Default Swaps [Member]
Notional value of derivative	150.3	148.5
Credit Default Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	8.1	4.3
Total Return Swaps [Member]
Notional value of derivative	65.8	69.7
Total Return Swaps [Member] \| Seeded Investment Products [Member]
Notional value of derivative	26.3	10.3
Interest Rate Swap [Member] \| Seeded Investment Products [Member]
Notional value of derivative	14.7	13.9
TBAs [Member]
Notional value of derivative	39.0	9.1
Other [Member] \| Seeded Investment Products [Member]
Notional value of derivative	$ 1.6	$ 0.0