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Note 1 - Summary of Significant Accounting Policies (Details Textual) |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 01, 2018 USD ($) | Jan. 12, 2017 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2019 USD ($)
Number of Branches |  |  | 15 |  |  |
Servicing Asset at Fair Value, Amount, Ending Balance |  |  | $ 59,400,000 | $ 50,400,000 |  |
Goodwill, Impairment Loss |  |  | 0 | 0 | $ 0 |
Assets, Total |  |  | 1,248,398,000 | 1,106,336,000 |  |
Liabilities, Total |  |  | $ 1,120,108,000 | 986,473,000 |  |
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 |  |  |
Accounting Standards Update 2016-01 [Member] |  |  |  |  |  |
Reclassification of Investment Securities Available for Sale to Equity Securities | $ 625,000 |  | $ 625,000 |  |  |
Reclassification of AOCI and Retained Earnings | 141,000 |  |  |  |  |
Accounting Standards Update 2018-02 [Member] |  |  |  |  |  |
Tax Cuts and Jobs Act of 2017 Reclassification From AOCI to Retained Earnings Tax Effect | $ 187,000 |  |  |  |  |
Accounting Standards Update 2016-02 [Member] | Scenario, Forecast [Member] |  |  |  |  |  |
Assets, Total |  |  |  |  |  | $ 6,700,000
Liabilities, Total |  |  |  |  |  | $ 6,700,000
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, Total |  | $ 42,168,000 |  |  |  |
Goodwill, Impairment Loss |  |  | $ 0 | $ 0 |  |

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