Note 2 - Mortgage-backed Securities and U.S. Treasury Securities, At Fair Value - Schedule of RMBS Portfolio (Details) - USD ($) $ in Thousands		Dec. 31, 2023		Dec. 31, 2022
Mortgage-backed securities, par value		$ 4,051,145		$ 3,848,367
Mortgage-backed securities, cost		4,220,088	[1]	4,112,707	[2]
Mortgage-backed securities		3,894,012		3,540,002
Pass Through Certificate [Member]
Mortgage-backed securities, par value		4,051,145		3,848,367
Mortgage-backed securities, cost		4,198,424	[1]	4,088,165	[2]
Mortgage-backed securities		3,877,082		3,519,906
Pass Through Certificate [Member] | Fixed Rate Mortgages [Member]
Mortgage-backed securities, par value		4,051,145		3,848,367
Mortgage-backed securities, cost		4,198,424	[1]	4,088,165	[2]
Mortgage-backed securities		3,877,082		3,519,906
Structured Finance [Member]
Mortgage-backed securities, par value		0		0
Mortgage-backed securities, cost		21,664	[1]	24,542	[2]
Mortgage-backed securities		16,930		20,096
Structured Finance [Member] | Interest Only Securities [Member]
Mortgage-backed securities, cost	[3]	19,839	[1]	22,395	[2]
Mortgage-backed securities	[3]	16,572		19,669
Structured Finance [Member] | Inverse Interest Only Securities [Member]
Mortgage-backed securities, cost	[4]	1,825	[1]	2,147	[2]
Mortgage-backed securities	[4]	$ 358		$ 427

[1] The cost information in the table above represents the aggregate current par value, multiplied by the purchase price of each security in the portfolio.

[2] The cost information in the table above represents the aggregate current par value, multiplied by the purchase price of each security in the portfolio.

[3] The notional balance for the interest-only securities portfolio was $98.6 million and $111.5 million as of December 31, 2023 and 2022, respectively.

[4] The notional balance for the inverse interest-only securities portfolio was $26.8 million and $31.5 million as of December 31, 2023 and 2022, respectively.