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Hedging Activities (Details) (USD $)
 1 Months Ended 3 Months Ended 1 Months Ended
Jun. 30, 2011
BasisPoints
Mar. 31, 2013
Dec. 31, 2012
Feb. 29, 2012
Feb. 29, 2012
Interest rate swap agreement [Member]
BasisPoints
Jun. 30, 2011
Interest rate cap contracts [Member]
Mar. 31, 2013
Mercantile Bank Prime Rate [Member]
Mar. 31, 2013
Wall Street Journal Prime Rate [Member]
Dec. 31, 2008
Wall Street Journal Prime Rate [Member]
Oct. 31, 2008
Wall Street Journal Prime Rate [Member]
Hedging Activities (Textual) [Abstract]                    
Prime Rate Percentage             4.50% 3.25%    
Decline in Prime Rate                 0.75% 0.50%
Notional amount of purchased interest rate cap   $ 18,800,000       $ 100,000,000        
Rate used to determine payment received from correspondent bank on notional amount of interest rate swap/Basis for determining strike rate on purchased interest rate cap 30-Day Libor Rate       90-Day Libor Rate          
Trust preferred securities interest rate index         90-Day Libor Rate          
Fixed spread added to the variable rate on our trust preferred securities         218          
Notional amount of interest rate swap         32,000,000          
Hedging Activities (Additional Textual) [Abstract]                    
Variable commercial loans as a percentage of total assets   38.00%                
Strike rate of sold interest rate cap in basis points above strike rate on purchased interest rate cap 125                  
Notional amount of sold interest rate cap 100,000,000                  
Payment made under interest rate cap contract   0                
Subordinated debentures       32,000,000            
Interest rate swap agreement effective date   2013-01                
Interest rate swap agreement maturity date   2018-01                
Present value of Interest rate swap, record as liability   $ 1,000,000 $ 1,100,000