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