Note 2 - Securities (Details Textual)	6 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 157,412,000	$ 253,210,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4,032,000	9,429,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	0
Available-for-sale Debt Securities, Amortized Cost Basis	323,548,000
Available-for-sale Securities	$ 322,258,000	328,060,000
Debt Securities [Member]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	180
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	$ 109,000,000	109,000,000
Available-for-sale Securities	110,000,000	107,000,000
All Other States and Their Political Subdivisions [Member]
Available-for-sale Debt Securities, Amortized Cost Basis	15,100,000	19,500,000
Available-for-sale Securities	$ 15,200,000	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	$ 111,000,000	$ 132,000,000