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Note 17 - Regulatory Capital (Tables)
 12 Months Ended
Dec. 31, 2019
Notes Tables  
Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block]
   
As of December 31, 2019
  
   
Leverage
    
Tier 1 Risk
Based
    
Common
Equity Tier 1
    
Total Risk
Based
  
The Middlefield Banking Company
    
10.35
 %    
12.12
 %    
12.12
 %    
12.79
 %
Middlefield Banc Corp.
    
10.23
 %    
12.56
 %    
11.77
 %    
13.23
 %
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, 2018
  
   
Leverage
    
Tier 1 Risk
Based
    
Common
Equity Tier 1
    
Total Risk
Based
  
The Middlefield Banking Company
    
9.60
 %    
11.09
 %    
11.09
 %    
11.83
 %
Middlefield Banc Corp.
    
10.35
 %    
11.83
 %    
11.04
 %    
12.57
 %
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
 %