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Hedging Activities (Details Textual) (USD $) | 1 Months Ended | 3 Months Ended | 6 Months Ended |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |
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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 |  |  |  |  |  |  |  |

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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 Basis point, current. + References No definition available. + Details
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- Definition Decline in prime rate. + References No definition available. + Details
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- Definition Fixed spread added to the variable rate on our trust preferred securities. + References No definition available. + Details
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- Details
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- Definition Mercantile bank prime rate, percentage of variable commercial loans. + References No definition available. + 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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- Definition Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 + Details
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