Note 2 - Securities (Details Textual)	9 Months Ended
Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 306,884,000	$ 252,521,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	17,365,000	7,559,000
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	343,434,000	341,948,000
Debt Securities, Available-for-sale, Total	326,531,000	335,744,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	95,700,000	112,000,000
Debt Securities, Available-for-sale, Total	94,100,000	112,000,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	9,600,000	12,900,000
Debt Securities, Available-for-sale, Total	$ 9,500,000	13,000,000
Securities Other Than US Government Agencies and the State of Michigan and Political Subdivisions [Member] \| Stockholders' Equity, Total [Member] \| Securities Issued Concentration Risk [Member]
Concentration Risk, Percentage	10.00%
US Government Agency Debt Obligations and Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale, Restricted	$ 112,000,000	$ 119,000,000
Debt Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	414

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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