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Note 14 - Employee Benefits (Details Textual) - USD ($) |  | 12 Months Ended
--- | --- | ---
May 10, 2017 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2007
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value |  | $ 166,759 | $ 251,068 |
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount |  | $ 0 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares) |  | 2,350 | 200 |
Deferred Compensation Share-based Arrangements, Liability, Current and Noncurrent |  | $ 581,000 | $ 694,000 |
Share-based Payment Arrangement, Option [Member] |  |  |  |
Payments For Stock Options Exercised |  | $ 18,000 | $ 0 |
Restricted Stock [Member] |  |  |  |
Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture, Total (in shares) |  | 23,648 | 29,130 |
Share-based Payment Arrangement, Expense |  | $ 271,000 | $ 184,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Average Total Shareholder Annual Return, Percentage |  | 10.00% | 10.00% |
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount |  | $ 272,000 |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) |  | 1 year 5 months 12 days |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value |  | $ 208,000 | $ 265,000 |
Retirement Plan [Member] |  |  |  |
Defined Contribution Plan Employer Voluntary Matching Bank Contribution |  | 50.00% | 50.00% |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  | 6.00% | 6.00% |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  | 6 years |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  | $ 350,000 | $ 319,000 |
Retirement Plan [Member] | Middlefield Banking Company [Member] |  |  |  |
Defined Contribution Plan Annual Vesting Percentage |  | 20.00% |  |
Retirement Plan [Member] | Liberty Bank, N.A. [Member] |  |  |  |
Deferred Compensation Arrangement with Individual, Requisite Service Period (Year) |  | 3 years |  |
Executive Deferred Compensation Plan [Member] | Middlefield Banking Company [Member] |  |  |  |
Deferred Compensation Arrangement with Individual, Recorded Liability |  | $ 1,700,000 | 1,500,000 |
Defined Contribution Plan, Employer Discretionary Contribution Amount |  | $ 240,000 | $ 260,000 |
Stock Option and Restricted Stock Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) |  |  |  | 320,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term (Year) |  |  |  | 10 years
Shares, Outstanding, Ending Balance (in shares) |  | 12,150 |  |
The 2017 Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) | 448,000 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term (Year) | 10 years |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) |  | 386,188 |  |
Omnibus Equity Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares) |  | 0 | 0 |
Share-based Payment Arrangement, Expensed and Capitalized, Amount, Total |  | $ 0 | $ 265,000 |

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