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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Segment
Dec. 31, 2012
Schedule Of Significant Accounting Policies [Line Items]    
Mercantile Bank Corporation incorporation date Jul. 15, 1997  
Mercantile Bank Corporation operational date Dec. 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%  
Mortgage company's dissolution date Jan. 01, 2013  
Maturities of short-term borrowings reported net for cash flow purposes 90 days or less  
Net unamortized deferred loan fees amounted $ 0.5 $ 0.7
Delinquent period on commercial and mortgage Loans 90 days  
Maximum period of Consumer and credit card loans 120 days  
Loans held for sale $ 1.1 $ 3.5
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  
Buildings and related components [Member] | Minimum [Member]
    
Schedule Of Significant Accounting Policies [Line Items]    
Property and Equipment Useful lives 5 years  
Buildings and related components [Member] | Maximum [Member]
    
Schedule Of Significant Accounting Policies [Line Items]    
Property and Equipment Useful lives 33 years  
Furniture and Fixtures [Member] | Minimum [Member]
    
Schedule Of Significant Accounting Policies [Line Items]    
Property and Equipment Useful lives 3 years  
Furniture and Fixtures [Member] | Maximum [Member]
    
Schedule Of Significant Accounting Policies [Line Items]    
Property and Equipment Useful lives 7 years  
Mercantile Bank Mortgage Company [Member]
    
Schedule 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]
    
Schedule 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]
    
Schedule Of Significant Accounting Policies [Line Items]    
Mercantile Bank Mortgage Company initiated business and Real Estate Jul. 21, 2003  
Date of facility operations Jun. 30, 2005