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Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2012 Segment | Dec. 31, 2010 derivative | Dec. 31, 2011
Summary of Significant Accounting Policies (Textual) [Abstract] |  |  |
Mercantile Bank Corporation incorporation date | Jul. 15, 1997 |  |
Date Mercantile Bank Mortgage Company was reorganized as Mercantile Bank Mortgage Company, LLC | Jan. 01, 2004 |  |
Percentage of ownership by the Bank for limited liability company | 99.00% |  |
Percentage of ownership interest distributed to insurance company | 1.00% |  |
Maturities of short-term borrowings reported net for cash flow purposes | 90 days or less |  |
Net unamortized deferred loan fees amounted | $ 0.7 |  | $ 0.4
Delinquent period on commercial and mortgage Loans | 90 days |  |
Maximum period of Consumer and credit card loans | 120 days |  |
Loans held for sale | $ 3.5 |  | $ 2.6
Reversed the full valuation allowance against the net deferred tax asset that was initially recorded at December 31, 2009 | Dec. 31, 2011 |  |
Number of operating Segments | 1 |  |
Derivatives outstanding |  | 0 |
Mercantile Bank Mortgage Company [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Bank subsidiary year of formation | 2000 |  |
Mercantile Bank Mortgage Company initiated business and Real Estate | Oct. 24, 2000 |  |
Mercantile Insurance Company [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Bank subsidiary year of formation | 2002 |  |
Ownership percentage of Mercantile Bank Mortgage Company, LLC | 1.00% |  |
Mercantile Real Estate [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Mercantile Bank Mortgage Company initiated business and Real Estate | Jul. 21, 2003 |  |
Buildings and related components [Member] | Maximum [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Property and Equipment Useful lives | 33 years |  |
Buildings and related components [Member] | Minimum [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Property and Equipment Useful lives | 5 years |  |
Furniture, fixtures and equipment [Member] | Maximum [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Property and Equipment Useful lives | 7 years |  |
Furniture, fixtures and equipment [Member] | Minimum [Member] |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Property and Equipment Useful lives | 3 years |  |

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- Definition Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Section 563c.102 -Paragraph 8 -Subparagraph a(i-v) -Chapter V -Subsection I -LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Section 563c.102 -Paragraph 8 -Subparagraph f -Chapter V -Subsection I -LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6378556&loc=d3e10152-111534 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Article 9 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5074-111524 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Section 563c.102 -Paragraph 8 -Subparagraph e -Chapter V -Subsection I -LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy. Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 13 -Paragraph 23 -Subparagraph a(i)(d) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 4 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=6952072&loc=d3e45377-112738 Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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