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Note 2 - Securities (Details Textual)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	209
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 298,725,000	$ 119,073,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	4,447,000	899,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance	0
Debt Securities, Available-for-sale, Amortized Cost, Total	504,219,000	380,403,000
Debt Securities, Available-for-sale, Total	506,125,000	387,347,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	141,000,000	109,000,000
Debt Securities, Available-for-sale, Total	145,000,000	116,000,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	2,700,000	4,100,000
Debt Securities, Available-for-sale, Total	2,800,000	4,200,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 170,000,000	$ 118,000,000

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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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