Hedging Activities (Details) (USD $)	1 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended
	Jun. 30, 2011 BasisPoints	Sep. 30, 2012	Sep. 30, 2012	Feb. 29, 2012	Feb. 29, 2012 Interest rate swap agreement [Member]	Sep. 30, 2012 Interest rate cap contracts [Member] Commercial Loans [Member]	Sep. 30, 2012 Interest rate cap contracts [Member] Commercial Loans [Member]	Sep. 30, 2012 Mercantile Bank Prime Rate [Member]	Sep. 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]
Mercantile Bank Prime Rate indexed loans as a percentage of aggregate loans								31.00%
Prime Rate Percentage								4.50%	3.25%
Decline in Prime Rate										0.75%	0.50%
Net reduction in interest income on commercial loans						$ 51,000	$ 241,000
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 percentage of total loans		59.00%	59.00%
Notional amount of purchased interest rate cap	100,000,000	30,000,000	30,000,000
Basis for determining strike rate on purchased interest rate cap	30-Day Libor Rate
Notional amount of sold interest rate cap	100,000,000
Strike rate of sold interest rate cap in basis points above strike rate on purchased interest rate cap	125
Payments made under interest rate cap contracts		0	0
Trust preferred securities				32,000,000
Fair value of Interest rate swap, record as liability		1,100,000	1,100,000
Fair value of interest rate swap on trade date				$ 0
Interest rate swap agreement maturity date			2018-01