Note 2 - Securities (Details Textual)	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 220,635,000		$ 253,210,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	8,586,000		9,429,000
Available-for-sale Debt Securities, Amortized Cost Basis	339,964,000
Available-for-sale Securities	332,441,000		328,060,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	0
Proceeds from Sale of Available-for-sale Securities	$ 0	$ 264,000
Debt Securities [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	316
Mutual Funds [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	1
State of Michigan and All Its Political Subdivisions [Member]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 113,000,000		109,000,000
Available-for-sale Securities	112,000,000		107,000,000
All Other States and Their Political Subdivisions [Member]
Available-for-sale Debt Securities, Amortized Cost Basis	18,700,000		19,500,000
Available-for-sale Securities	$ 18,800,000		19,500,000
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]
Available-for-sale Securities, Restricted	$ 127,000,000		132,000,000
Municipal General Obligation Bonds [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	55,533,000		93,629,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,201,000		1,549,000
Available-for-sale Securities	122,530,000		$ 119,047,000
Proceeds from Sale of Available-for-sale Securities	$ 0	$ 0