Note 2 - Mortgage-backed Securities and U.S. Treasury Notes - Schedule of RMBS Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	$ 3,540,002	$ 6,511,095
Pass Through Certificate [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	3,519,906	6,298,189
Pass Through Certificate [Member] | Fixed Rate Mortgages [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	3,519,906	6,298,189
Structured Finance [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	20,096	212,906
Structured Finance [Member] | Interest Only Securities [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	19,669	210,382
Structured Finance [Member] | Inverse Interest Only Securities [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $6,506,372 and $3,719,906, respectively)	$ 427	$ 2,524