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Note 17 - Regulatory Capital (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block]
   
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
 %
   
As of December 31, 2016
  
           
Tier 1 Risk
    
Common
    
Total Risk
  
   
Leverage
     Based     Equity Tier 1     Based  
The Middlefield Banking Company
    
9.29
 %    
13.03
 %    
13.03
 %    
14.25
 %
Middlefield Banc Corp.
    
9.27
 %    
13.07
 %    
13.07
 %    
15.75
 %
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
 %