Note 7 - Investment and Equity Securities (Details Textual) - USD ($)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Security Owned and Pledged as Collateral, Fair Value, Total	$ 61,700,000		$ 57,900,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Total	82
Available-for-sale Securities, Percentage of Portfolio	100.00%
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held, Ending Balance	$ 0
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Total	$ 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

Represents a percentage of the available-for-sale securities portfolio.

No definition available.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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In a business combination achieved in stages, this element represents the amount of gain recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination.

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- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

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- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

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