Note 2 - Securities (Details Textual) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	679	641
Debt Securities, Available-for-Sale, Unrealized Loss Position	$ 556,083	$ 548,747
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	68,460	66,112
Proceeds from Sale of Debt Securities, Available-for-Sale	0	0
Debt Securities, Available-for-Sale, Amortized Cost	676,208	680,998
Debt Securities, Available-for-Sale	609,153	617,092
Bonds Issued or Guaranteed by Agencies of the U.S. Federal Government [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	59,200
Bonds Issued by State-based Municipalities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	9,200
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-Sale, Amortized Cost	203,000	203,000
Debt Securities, Available-for-Sale	195,000	197,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-Sale, Amortized Cost	0	0
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Restricted	$ 229,000	$ 230,000

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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