Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016	Jan. 01, 2019 USD ($)	Jun. 01, 2014 USD ($)
Number of Unconsolidated Business Trusts Formed to Issue Trust Preferred Securities	5
Number of Operating Segments	1
Loans and Leases Receivable, Deferred Income, Total	$ 0.9	$ 1.0
Interest Income on Commercial and Mortgage Loans Discontinued Period	90 days
Maximum Delinquency Period for Consumer and Credit Card Loans to be Charged Off	120 days
Repossessed Assets, Total	$ 0.8	$ 2.3
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%	35.00%
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Operating Lease, Right-of-Use Asset				$ 1.3
Operating Lease, Liability, Total				$ 1.3
Adjustments for New Accounting Principle, Early Adoption [Member]
Reclassification from AOCI to Retained Earnings, Current Period		$ 0.9
Core Deposits [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	5 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	33 years
Furniture and Fixtures [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life	3 years
Furniture and Fixtures [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life	7 years
Firstbank Corporation [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total					$ 1,500.0
Business Combination Branches Acquired					46