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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

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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 Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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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 Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714 + Details
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