Note 2 - Securities (Details Textual)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	123
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 112,880,000	$ 278,331,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	1,703,000	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	344,342,000	347,795,000
Debt Securities, Available-for-sale, Total	347,924,000	337,366,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	91,200,000	98,200,000
Debt Securities, Available-for-sale, Total	93,500,000	97,400,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	7,500,000	8,700,000
Debt Securities, Available-for-sale, Total	7,500,000	8,700,000
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 120,000,000	$ 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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