Note 2 - Securities (Details Textual)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	185	64
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 262,853,000	$ 119,073,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	8,331,000	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	436,481,000	380,403,000
Debt Securities, Available-for-sale, Total	434,257,000	387,347,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	123,000,000	109,000,000
Debt Securities, Available-for-sale, Total	128,000,000	116,000,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	3,500,000	4,100,000
Debt Securities, Available-for-sale, Total	3,600,000	4,200,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 141,000,000	$ 118,000,000

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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