Note 2 - Securities (Details Textual)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	94
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 91,774,000	$ 278,331,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	1,097,000	11,357,000
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	340,141,000	347,795,000
Debt Securities, Available-for-sale, Total	345,533,000	337,366,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	94,300,000	98,200,000
Debt Securities, Available-for-sale, Total	97,000,000	97,400,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	6,900,000	8,700,000
Debt Securities, Available-for-sale, Total	6,900,000	8,700,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 104,000,000	$ 104,000,000

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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