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Note 9 - Business Acquisition (Details Textual) - USD ($)
3 Months Ended 6 Months Ended
Jan. 12, 2017
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Goodwill   $ 15,435,000     $ 15,435,000   $ 4,559,000
Business Combination, Acquisition Related Costs   307,000   694,000  
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member]              
Business Acquisition, Percentage of Voting Interests Acquired       100.00%   100.00%  
Business Combination, Consideration Transferred $ 42,168,000            
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares 544,610            
Payments to Acquire Businesses, Gross $ 21,173,000            
Shares Issued, Price Per Share $ 38.55            
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain     $ 488,000   488,000    
Goodwill $ 10,876,000            
Goodwill, Impairment Loss         0    
Goodwill, Purchase Accounting Adjustments         211,000    
Business Combination, Acquisition Related Costs         467,000    
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member] | Core Deposits [Member]              
Finite-Lived Core Deposits, Gross   2,932,000     2,932,000    
Finite-Lived Intangible Assets, Accumulated Amortization   $ 155,000     $ 155,000