Note 2 - Securities (Details Textual) $ in Thousands	Sep. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions	1,240	607
Debt Securities, Available-for-Sale, Unrealized Loss Position	$ 1,084,845	$ 545,301
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	78,206	36,856
Debt Securities, Available-for-Sale, Amortized Cost	1,176,200	1,132,603
Debt Securities, Available-for-Sale	1,098,016	1,102,230
Bonds Issued or Guaranteed by Agencies of the U.S. Federal Government [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	59,600
Bonds Issued by State-based Municipalities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	18,100
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-Sale, Amortized Cost	250,000	252,000
Debt Securities, Available-for-Sale	233,000	249,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-Sale, Amortized Cost	29,100	31,300
Debt Securities, Available-for-Sale	28,400	31,300
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-Sale, Restricted	197,000	232,000
US Government Agencies Debt Securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position	680,037	392,306
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss	47,056	26,411
Debt Securities, Available-for-Sale, Amortized Cost	729,057	661,841
Debt Securities, Available-for-Sale	682,002	639,773
Debt Securities, Available-for-Sale, Restricted	$ 12,100	$ 12,600

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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