Note 2 - Mortgage-backed Securities and U.S. Treasury Notes - Gain (Loss) on Investments (Details) - Collateralized Mortgage-Backed Securities [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Proceeds from sales of RMBS	$ 2,759,919	$ 2,851,708	$ 4,200,536
Carrying value of RMBS sold	(2,893,614)	(2,857,250)	(4,225,522)
Realized losses on mortgage-backed securities	(133,695)	(5,542)	(24,986)
Gross gain on sales of RMBS	2,705	7,930	8,678
Gross loss on sales of RMBS	$ (136,400)	$ (13,472)	$ (33,664)