Note 6 - Investment Securities Available for Sale (Details Textual) - USD ($)	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Available-for-sale Securities Pledged as Collateral	$ 64,900,000	$ 60,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	28
Available-for-sale Securities, Percentage of Portfolio	99.00%
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 0		$ 0
Pledged to Secure Deposits and Other Purposes Required by Law [Member]
Restricted Cash	4,500,000
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.

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- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

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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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- Definition

Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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