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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%