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Note 4 - Investment and Equity Securities (Details Textual) - USD ($)			12 Months Ended
		Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017
Equity Securities, FV-NI			$ 616,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	[1]		(9,000)
Equity Securities, FV-NI, Realized Gain (Loss), Total			0
Security Owned and Pledged as Collateral, Fair Value, Total			63,500,000	57,900,000
Proceeds from Sale of Available-for-sale Securities, Total			$ 0	6,474,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Total			85
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
Merger of Liberty Bank, N.A. into The Middlefield Banking Company [Member]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain				$ 488,000
Accounting Standards Update 2016-01 [Member]
Reclassification of Investment Securities Available for Sale to Equity Securities		$ 625,000	$ 625,000
[1] Not within scope of ASC 606

X

- Definition

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

+ References

No definition available.

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- Definition

The amount of reclassification of investment securities available for sale to equity securities.

+ References

No definition available.

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- Definition

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.

+ References

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-Publisher FASB
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-Topic 320
-SubTopic 10
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- Definition

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.

+ References

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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
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- Definition

Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

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-Publisher FASB
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-SubTopic 10
-Section 50
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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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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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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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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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