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Note 15 - Regulatory Restrictions (Detail) (USD $) | 0 Months Ended | 1 Months Ended | 12 Months Ended |  | 12 Months Ended |  |  | 3 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 30, 2012 | Apr. 17, 2012 | Dec. 31, 2006 | Dec. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2012 Subsequent Event [Member] Pursuant to Regulatory Clearance [Member] One Investor [Member] | Dec. 31, 2012 Subsequent Event [Member] One Investor [Member] | Dec. 31, 2012 Subsequent Event [Member] | Dec. 31, 2011 Minimum Required for FDIC Memorandum of Understanding [Member] Emerald Bank [Member] | Dec. 31, 2011 Minimum Required for FDIC Memorandum of Understanding [Member] Middlefield Banking Company [Member] | Jun. 30, 2010 Minimum Required for FDIC Memorandum of Understanding [Member] Middlefield Banking Company [Member] | Dec. 31, 2009 Emerald Bank [Member] | Dec. 31, 2012 Emerald Bank [Member] | Dec. 31, 2011 Emerald Bank [Member] | Feb. 28, 2011 Emerald Bank [Member] | Dec. 31, 2010 Emerald Bank [Member] | Dec. 31, 2012 Middlefield Banking Company [Member] | Dec. 31, 2011 Middlefield Banking Company [Member] | Dec. 31, 2012 One Investor [Member]
Tier One Leverage Capital Required for Capital Adequacy to Average Assets |  |  |  |  |  |  |  |  |  | 7.25% |  |  | 4.00% | 4.00% | 9.00% |  | 4.00% | 4.00% |
Nonperforming Assets Sold (in Dollars) |  |  |  |  |  |  |  |  |  |  |  | $ 5,800,000 |  |  |  |  |  |  |
Advances to Affiliate (in Dollars) |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000 |  | 500,000 |  |  |
Tier One Leverage Capital to Average Assets |  |  |  |  |  |  |  |  | 10.61% |  | 6.25% |  | 10.61% | 9.92% |  |  | 7.32% | 6.75% |
Tier One Risk Based Capital to Risk Weighted Assets |  |  |  |  |  |  |  |  |  |  | 11.29% |  | 14.16% | 12.57% |  |  | 12.04% | 10.50% |
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets |  |  |  |  |  |  |  |  |  | 12.00% |  |  | 4.00% | 4.00% |  |  | 4.00% | 4.00% |
Reserves Required by Federal Reserve Bank (in Dollars) |  |  |  | 5,419,000 | 7,063,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount Available for Payment of Dividends (in Dollars) |  |  |  |  |  |  |  |  |  |  |  |  | $ 144,000 |  |  |  | $ 8,077,000 |  |
Stock Issued During Period, Shares, New Issues (in Shares) | 103,585 | 93,050 | 248,000 | 138,150 |  |  | 196,635 | 13,320 |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Parent |  |  |  |  |  |  | 9.90% |  |  |  |  |  |  |  |  |  |  |  |
Maximum Ownership of Stock |  |  |  |  |  | 24.99% |  |  |  |  |  |  |  |  |  |  |  |  | 10.00%

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