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Note 6 - Debt Securities - Securities Available-for-sale Continuous Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Less than 12 months, estimated fair value $ 23,819 $ 406,076
Less than 12 months, unrealized losses (937) (31,483)
12 months or more, estimated fair value 710,255 367,850
12 months or more, unrealized losses (86,897) (52,117)
Total estimated fair value 734,074 773,926
Total unrealized losses (87,834) (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)
12 months or more, estimated fair value 202,522 147,215
12 months or more, unrealized losses (18,510) (15,508)
Total estimated fair value 202,522 205,097
Total unrealized losses (18,510) (19,468)
US Government Agencies Debt Securities [Member]    
Less than 12 months, estimated fair value 1,934 61,821
Less than 12 months, unrealized losses (64) (4,293)
12 months or more, estimated fair value 95,544 38,492
12 months or more, unrealized losses (9,486) (5,148)
Total estimated fair value 97,478 100,313
Total unrealized losses (9,550) (9,441)
Mortgage-Backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Less than 12 months, estimated fair value 816 45,440
Less than 12 months, unrealized losses (34) (4,393)
12 months or more, estimated fair value 100,453 70,854
12 months or more, unrealized losses (18,416) (12,075)
Total estimated fair value 101,269 116,294
Total unrealized losses (18,450) (16,468)
US States and Political Subdivisions Debt Securities [Member]    
Less than 12 months, estimated fair value 17,036 181,640
Less than 12 months, unrealized losses (662) (14,556)
12 months or more, estimated fair value 246,247 97,907
12 months or more, unrealized losses (33,007) (16,647)
Total estimated fair value 263,283 279,547
Total unrealized losses (33,669) (31,203)
Corporate Debt Securities [Member]    
Less than 12 months, estimated fair value 4,033 59,293
Less than 12 months, unrealized losses (177) (4,281)
12 months or more, estimated fair value 65,489 13,382
12 months or more, unrealized losses (7,478) (2,739)
Total estimated fair value 69,522 72,675
Total unrealized losses $ (7,655) $ (7,020)