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Note 20 - Subordinated Debentures (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Schedule of Subordinated Borrowing [Table Text Block]
   
Preferred
Securities
        
Trust Name
  
Outstanding
  
Interest Rate
  
Maturity Date
               
Mercantile Bank Capital Trust I
   $
21,000,000
  
3 Month Libor + 218 bps
  
September 16, 2034
               
Firstbank Capital Trust I
   $
10,000,000
  
3 Month Libor + 199 bps
  
October 18, 2034
               
Firstbank Capital Trust II
   $
10,000,000
  
3 Month Libor + 127 bps
  
April 7, 2036
               
Firstbank Capital Trust III
   $
7,500,000
  
3 Month Libor + 135 bps
  
July 30, 2037
               
Firstbank Capital Trust IV
   $
7,500,000
  
3 Month Libor + 135 bps
  
July 30, 2037