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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 |  |

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