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Note 1 - Summary of Significant Accounting Policies (Details Textual)
12 Months Ended
Jan. 12, 2017
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Number of Branches   14    
Servicing Asset at Fair Value, Amount   $ 50,400,000 $ 39,900,000  
Goodwill, Impairment Loss   $ 0 $ 0 $ 0
Core Deposits [Member]        
Finite-Lived Intangible Asset, Useful Life   10 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   20 years    
Building and Building Improvements [Member] | Minimum [Member]        
Property, Plant and Equipment, Useful Life   3 years    
Building and Building Improvements [Member] | Maximum [Member]        
Property, Plant and Equipment, Useful Life   40 years    
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member]        
Business Combination, Merger Agreement, Cash Per Share of Reporting Entity Common Stock | $ / shares $ 37.96      
Business Combination, Merger Agreement, Ratio of Shares of Reporting Entity's Common Stock for Each Share of Acquired Entity's Common Stock | shares 1.1934      
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares 544,610      
Business Combination, Consideration Transferred $ 42,168,000      
Goodwill, Impairment Loss   $ 0