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Note 17 - Regulatory Capital (Tables)
12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block]
   
As of December 31, 2018
 
           
Tier 1 Risk
   
Common
   
Total Risk
 
   
Leverage
    Based     Equity Tier 1     Based  
The Middlefield Banking Company
   
9.60
%    
11.09
%    
11.09
%    
11.83
%
Middlefield Banc Corp.
   
10.55
%    
10.35
%    
9.65
%    
11.00
%
Adequately capitalized ratio
   
4.00
%    
6.00
%    
4.50
%    
8.00
%
Adequately capitalized ratio plus fully phased-in capital conservation buffer
   
4.00
%    
8.50
%    
7.00
%    
10.50
%
Well-capitalized ratio (Bank only)
   
5.00
%    
8.00
%    
6.50
%    
10.00
%
   
As of December 31, 2017
 
           
Tier 1 Risk
   
Common
   
Total Risk
 
   
Leverage
    Based     Equity Tier 1     Based  
The Middlefield Banking Company
   
9.47
%    
10.88
%    
10.88
%    
11.64
%
Middlefield Banc Corp.
   
10.20
%    
11.64
%    
10.79
%    
12.41
%
Adequately capitalized ratio
   
4.00
%    
6.00
%    
4.50
%    
8.00
%
Adequately capitalized ratio plus fully phased-in capital conservation buffer
   
4.00
%    
8.50
%    
7.00
%    
10.50
%
Well-capitalized ratio (Bank only)
   
5.00
%    
8.00
%    
6.50
%    
10.00
%