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Note 7 - Investment and Equity Securities (Details Textual)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Equity Securities without Readily Determinable Fair Value, Number of Securities 1   1    
Equity Securities without Readily Determinable Fair Value, Amount $ 250,000   $ 250,000    
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Cumulative Amount 0   0    
Equity Securities, FV-NI, Unrealized Gain (Loss), Total [1] 13,000 31,000  
Equity Securities, FV-NI, Realized Gain (Loss), Total 0   0    
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Total     0 0  
Security Owned and Pledged as Collateral, Fair Value, Total $ 67,900,000   $ 67,900,000   $ 57,900,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Total 87   87    
Available-for-sale Securities, Percentage of Portfolio 100.00%   100.00%    
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance $ 0   $ 0    
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member]          
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain       $ 488,000  
[1] Not within scope of ASC 606.