Note 2 - Mortgage-backed Securities and U.S. Treasury Notes - Schedule of RMBS Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	$ 3,999,906	$ 3,540,002
Pass Through Certificate [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	3,980,462	3,519,906
Pass Through Certificate [Member] | Fixed Rate Mortgages [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	3,980,462	3,519,906
Structured Finance [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	19,444	20,096
Structured Finance [Member] | Interest Only Securities [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	18,962	19,669
Structured Finance [Member] | Inverse Interest Only Securities [Member]
Mortgage-backed securities, at fair value (includes pledged assets of $3,946,156 and $3,512,640, respectively)	$ 482	$ 427