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Note 3 - Debt Securities - Securities Available-for-sale Continuous Unrealized Loss Position (Details)
Pure in Thousands, $ in Thousands
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Less than 12 months, estimated fair value $ 14,624 $ 406,076
Less than 12 months, unrealized losses $ (204) $ (31,483)
Less than 12 months, securities 28 606
12 months or more, estimated fair value $ 706,936 $ 367,850
12 months or more, unrealized losses $ (62,245) $ (52,117)
12 months or more, securities 912 404
Total estimated fair value $ 721,560 $ 773,926
Total unrealized losses (62,449) (83,600)
US Treasury Securities [Member]    
Less than 12 months, estimated fair value 0 57,882
Less than 12 months, unrealized losses $ 0 $ (3,960)
Less than 12 months, securities 0 45
12 months or more, estimated fair value $ 196,432 $ 147,215
12 months or more, unrealized losses $ (13,587) $ (15,508)
12 months or more, securities 121 89
Total estimated fair value $ 196,432 $ 205,097
Total unrealized losses (13,587) (19,468)
US Government Agencies Debt Securities [Member]    
Less than 12 months, estimated fair value 1,986 61,821
Less than 12 months, unrealized losses $ (11) $ (4,293)
Less than 12 months, securities 2 51
12 months or more, estimated fair value $ 90,137 $ 38,492
12 months or more, unrealized losses $ (6,831) $ (5,148)
12 months or more, securities 78 32
Total estimated fair value $ 92,123 $ 100,313
Total unrealized losses (6,842) (9,441)
Mortgage-Backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Less than 12 months, estimated fair value 467 45,440
Less than 12 months, unrealized losses $ (12) $ (4,393)
Less than 12 months, securities 4 104
12 months or more, estimated fair value $ 101,265 $ 70,854
12 months or more, unrealized losses $ (14,112) $ (12,075)
12 months or more, securities 155 57
Total estimated fair value $ 101,732 $ 116,294
Total unrealized losses (14,124) (16,468)
US States and Political Subdivisions Debt Securities [Member]    
Less than 12 months, estimated fair value 9,054 181,640
Less than 12 months, unrealized losses $ (73) $ (14,556)
Less than 12 months, securities 18 335
12 months or more, estimated fair value $ 251,286 $ 97,907
12 months or more, unrealized losses $ (22,604) $ (16,647)
12 months or more, securities 474 203
Total estimated fair value $ 260,340 $ 279,547
Total unrealized losses (22,677) (31,203)
Corporate Debt Securities [Member]    
Less than 12 months, estimated fair value 3,117 59,293
Less than 12 months, unrealized losses $ (108) $ (4,281)
Less than 12 months, securities 4 71
12 months or more, estimated fair value $ 67,816 $ 13,382
12 months or more, unrealized losses $ (5,111) $ (2,739)
12 months or more, securities 84 23
Total estimated fair value $ 70,933 $ 72,675
Total unrealized losses $ (5,219) $ (7,020)