Note 14 - Employee Benefits (Details Textual) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 538,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	8,237	500
Payments For Stock Options Exercised	$ (184,000)	$ 6,000	$ 7,000
Restricted Stock [Member]
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures	0	0	0
Employee Stock Option [Member]
Payments For Stock Options Exercised	$ 95,000	$ 6,000
Restricted Stock Units (RSUs) [Member]
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures	5,825	5,090	3,905
Allocated Share-based Compensation Expense	$ 196,000	$ 123,000	$ 55,000
Retirement Plan [Member]
Defined Contribution Plan Employer Voluntary Matching Bank Contribution	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	6 years
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 258,000	156,000	156,000
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	3 years
Directors Retirement Plan [Member] \| Middlefield Banking Company [Member]
Deferred Compensation Arrangement with Individual, Requisite Service Period	5 years
Retirement Benefits Term	10 years
Postretirement Benefits Percentage of Compensation	25.00%
Executive Deferred Compensation Plan [Member] \| Middlefield Banking Company [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 110,000	$ 99,000	$ 65,000
Stock Option and Restricted Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	160,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	10 years
Shares, Outstanding	92,759
The 2017 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	224,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	10 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	218,175
Omnibus Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0

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The percentage of matching voluntary contributions that the bank bases its contributions to the plan on.

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Postretirement benefits percentage of compensation.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares issued which are neither cancelled nor held in the treasury.

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Number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP), net of any shares forfeited. Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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