Note 2 - Mortgage-backed Securities and U.S. Treasury Securities, At Fair Value - Gain (Loss) on Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Realized losses on mortgage-backed securities	$ (22,642)	$ (133,695)	$ (5,542)
Collateralized Mortgage-Backed Securities [Member]
Proceeds from sales of RMBS	797,633	2,759,919	2,851,708
Carrying value of RMBS sold	(820,275)	(2,893,614)	(2,857,250)
Realized losses on mortgage-backed securities	(22,642)	(133,695)	(5,542)
Gross gain on sales of RMBS	0	2,705	7,930
Gross loss on sales of RMBS	$ (22,642)	$ (136,400)	$ (13,472)