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Hedging Activities (Details) (USD $) | 1 Months Ended | 12 Months Ended |  |  | 1 Months Ended |  | 12 Months Ended |  |  |  |  |
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Jun. 30, 2011 BasisPoints | Dec. 31, 2012 | Mar. 31, 2012 | Feb. 29, 2012 | Feb. 29, 2012 Interest rate swap agreement [Member] BasisPoints | Jun. 30, 2011 Interest rate cap contracts [Member] | Dec. 31, 2012 Interest rate cap contracts [Member] Commercial Loans [Member] | Dec. 31, 2012 Mercantile Bank Prime Rate [Member] | Mar. 31, 2012 Mercantile Bank Prime Rate [Member] | Dec. 31, 2012 Wall Street Journal Prime Rate [Member] Indices | 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%
Number of indices which likely to remain in the future period |  |  |  |  |  |  |  |  |  | 2 |  |
Variable commercial loans percentage |  |  | 47.00% |  |  |  |  |  | 64.00% |  |  |
Notional amount of purchased interest rate cap |  | $ 29,700,000 |  |  |  | $ 100,000,000 |  |  |  |  |  |
Bank payment on corresponding notional amount fixed rate/ Basis for determining strike rate on purchased interest rate cap | 30-Day Libor Rate |  |  |  | 90-Day Libor Rate |  |  |  |  |  |  |
Net reduction in interest income on commercial loans |  |  |  |  |  |  | 254,000 |  |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |
Present value of purchased interest rate cap on settlement date, asset | 729,500 |  |  |  |  |  |  |  |  |  |  |
Present value of sold interest rate cap on settlement date, liability | 213,500 |  |  |  |  |  |  |  |  |  |  |
Payment made under interest rate cap contract |  | 0 |  |  |  |  |  |  |  |  |  |
Trust preferred securities |  |  |  | 32,000,000 |  |  |  |  |  |  |  |
Interest rate swap agreement maturity date |  | 2018-01 |  |  |  |  |  |  |  |  |  |
Fair value of Interest rate swap, record as liability |  | 1,100,000 |  |  |  |  |  |  |  |  |  |
Fair value of interest rate swap on trade date |  |  |  | $ 0 |  |  |  |  |  |  |  |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition The reference rate for the variable rate of the interest rate derivative, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
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