Hedging Activities (Details Textual) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended		1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2012	Feb. 29, 2012	Feb. 29, 2012 Interest rate swap agreement [Member] BasisPoint	Jun. 30, 2012 Interest rate cap contracts [Member] Commercial loans [Member]	Jun. 30, 2012 Interest rate cap contracts [Member] Commercial loans [Member]	Jun. 30, 2012 Mercantile Bank Prime Rate [Member]	Jun. 30, 2012 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]
Variable commercial loans percentage of total loans		46.00%	46.00%
Mercantile Bank Prime Rate, Percentage of Variable Commercial Loans		37.00%	37.00%
Prime Rate Percentage								4.50%	3.25%
Decline in Prime Rate										0.75%	0.50%
Trust preferred securities interest rate index					90-Day Libor Rate
Notional amount of purchased interest rate cap	$ 100,000,000	$ 30,400,000	$ 30,400,000
Interest income on commercial loans						37,000	191,000
Fixed spread added to the variable rate on our trust preferred securities					218
Trust preferred securities				32,000,000
Notional amount of interest rate swap					32,000,000
Present value of interest rate swap, recorded as a liability				600,000
Present value of interest rate swap on trade date				0
Basis for determining strike rate on purchased interest rate cap	30-Day Libor Rate
Strike rate of sold interest rate cap in basis points above strike rate on purchased interest rate cap	1.25%
Notional amount of sold interest rate cap	100,000,000
Payments made under interest rate cap contracts		$ 0	$ 0