Note 2 - Securities (Details Textual)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	38
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 68,034,000	$ 117,573,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	515,000	1,254,000
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	305,396,000	329,979,000
Debt Securities, Available-for-sale, Total	312,424,000	334,655,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	102,000,000	96,500,000
Debt Securities, Available-for-sale, Total	107,000,000	99,400,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	4,400,000	5,900,000
Debt Securities, Available-for-sale, Total	4,500,000	5,900,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	157,000,000	$ 103,000,000
Maximum [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 500,000

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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