Note 2 - Business Combination (Details Textual)	Jun. 01, 2014 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Goodwill, Ending Balance		$ 49,473,000	$ 49,473,000
Firstbank Corporation [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Total	$ 1,500,000,000
Business Combination Branches Acquired	46
Business Acquisition Conversion of Stock Per Share | shares	1
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares	8,087,272
Goodwill, Ending Balance	$ 49,500,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount	$ 0