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Note 2 - Securities (Details Textual)	3 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	$ 296,119,000	$ 252,521,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	14,039,000	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	350,335,000	341,948,000
Debt Securities, Available-for-sale, Total	336,988,000	335,744,000
State of Michigan and All Its Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	112,000,000	112,000,000
Debt Securities, Available-for-sale, Total	111,000,000	112,000,000
All Other States and Their Political Subdivisions [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	10,800,000	12,900,000
Debt Securities, Available-for-sale, Total	$ 10,700,000	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	$ 105,000,000	$ 119,000,000
Debt Securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	394
Mutual Funds [Member]
Equity Securities, Number of Positions	1

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