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Note 9 - Business Acquisition (Details Textual) - USD ($)
3 Months Ended 9 Months Ended
Jan. 12, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Jun. 30, 2016
Goodwill   $ 15,299,000   $ 15,299,000 $ 15,299,000   $ 4,559,000  
Business Combination, Acquisition Related Costs   338,000 1,032,000      
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member]                
Business Acquisition, Percentage of Voting Interests Acquired               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        
Goodwill $ 10,740,000              
Goodwill, Impairment Loss       0        
Goodwill, Purchase Accounting Adjustments         347,000      
Business Combination, Acquisition Related Costs   338,000   1,032,000        
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member] | Core Deposits [Member]                
Finite-Lived Core Deposits, Gross   2,838,000   2,838,000 2,838,000      
Finite-Lived Intangible Assets, Accumulated Amortization   $ 249,000   $ 249,000 $ 249,000