Note 5 - Derivative Instruments - Change in Fair Value of Derivatives (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Gain (losses) from derivative instruments	$ 1.6	$ 51.2	$ (8.5)	$ 69.3
Futures [Member]
Gain (losses) from derivative instruments	3.6	21.6	(3.1)	37.0
Credit Default Swaps [Member]
Gain (losses) from derivative instruments	(0.6)	3.3	(2.5)	5.6
Total Return Swaps [Member]
Gain (losses) from derivative instruments	(5.7)	28.8	(11.7)	36.2
Foreign Currency, Forward Contracts and Swaps [Member]
Gain (losses) from derivative instruments	$ 4.3	$ (2.5)	$ 8.8	$ (9.5)