Note 2 - Mortgage-backed Securities and U.S. Treasury Notes - Gain (Loss) on Investments (Details) - Collateralized Mortgage-Backed Securities [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Proceeds from sales of RMBS			$ 0	$ 1,934,606
Carrying value of RMBS sold			0	(2,001,135)
Realized losses on mortgage-backed securities	$ 0	$ (15,443)	0	(66,529)
Gross gain on sales of RMBS			0	2,705
Gross loss on sales of RMBS			$ 0	$ (69,234)