SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
SIGNIFICANT ACCOUNTING POLICIES (Policies)	Sep. 30, 2013
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: The unaudited financial statements for the three and nine months ended September 30, 2013 include the consolidated results of operations of Mercantile Bank Corporation and its consolidated subsidiaries. These subsidiaries include Mercantile Bank of Michigan (“our bank”) and our bank’s two subsidiaries, Mercantile Bank Real Estate Co., LLC (“our real estate company”) and Mercantile Insurance Center, Inc. (“our insurance center”). These consolidated financial statements have been prepared in accordance with the instructions for Form 10-Q and Item 303(b) of Regulation S-K and do not include all disclosures required by accounting principles generally accepted in the United States of America for a complete presentation of our financial condition and results of operations. In the opinion of management, the information reflects all adjustments (consisting only of normal recurring adjustments) which are necessary in order to make the financial statements not misleading and for a fair presentation of the results of operations for such periods. The results for the period ended September 30, 2013 should not be considered as indicative of results for a full year. For further information, refer to the consolidated financial statements and footnotes included in our annual report on Form 10-K for the year ended December 31, 2012. We formed a business trust, Mercantile Bank Capital Trust I (“the trust”), in 2004 to issue trust preferred securities. We issued subordinated debentures to the trust in return for the proceeds raised from the issuance of the trust preferred securities. The trust is not consolidated, but instead we report the subordinated debentures issued to the trust as a liability
Earnings Per Share	Earnings Per Share: Basic earnings per share is based on the weighted average number of common shares and participating securities outstanding during the period. Diluted earnings per share include the dilutive effect of additional potential common shares issuable under our stock-based compensation plans and our common stock warrant granted to the U.S. Department of Treasury that we repurchased on July 3, 2012, and are determined using the treasury stock method. Our unvested restricted shares, which contain non-forfeitable rights to dividends whether paid or accrued (i.e., participating securities), are included in the number of shares outstanding for both basic and diluted earnings per share calculations. In the event of a net loss, our unvested restricted shares are excluded from the calculation of both basic and diluted earnings per share. Approximately 64,000 unvested restricted shares were included in determining both basic and diluted earnings per share for the three and nine months ended September 30, 2013. In addition, stock options for approximately 54,000 shares of common stock were included in determining diluted earnings per share for the three and nine months ended September 30, 2013. Stock options for approximately 94,000 shares of common stock were antidilutive and not included in determining diluted earnings per share for the three and nine months ended September 30, 2013. Approximately 38,000 unvested restricted shares were included in determining both basic and diluted earnings per share for the three and nine months ended September 30, 2012. In addition, stock options for approximately 26,000 and 21,000 shares of common stock were included in determining diluted earnings per share for the three and nine months ended September 30, 2012, respectively. Also, our stock warrant for approximately 616,000 shares of common stock was included in determining diluted earnings per share for the three and nine months ended September 30, 2012 taking into account our full repurchase of said stock warrant on July 3, 2012. Stock options for approximately 137,000 and 142,000 shares of common stock were antidilutive and not included in determining diluted earnings per share for the three and nine months ended September, 30, 2012, respectively.
Loans	Loans: Loans that we have the intent and ability to hold for the foreseeable future or until maturity or payoff are reported at the principal balance outstanding, net of deferred loan fees and costs and an allowance for loan losses. Interest income is accrued on the unpaid principal balance. Loan origination fees, net of certain direct origination costs, are deferred and recognized in interest income using the level-yield method without anticipating prepayments. Interest income on commercial loans and mortgage loans is discontinued at the time the loan is 90 days delinquent unless the loan is well-secured and in process of collection. Consumer and credit card loans are typically charged off no later than when they are 120 days past due. Past due status is based on the contractual terms of the loan. In all cases, loans are placed on nonaccrual or charged off at an earlier date if collection of principal and interest is considered doubtful. All interest accrued but not received for loans placed on nonaccrual is reversed against interest income. Interest received on such loans is accounted for on the cash-basis or cost-recovery method, until qualifying for return to accrual. Loans are returned to accrual status when all the principal and interest amounts contractually due are brought current and future payments are reasonably assured.
Allowance for Loan Losses	Allowance for Loan Losses: The allowance for loan losses (“allowance”) is a valuation allowance for probable incurred credit losses. Loan losses are charged against the allowance when we believe the uncollectability of a loan is confirmed. Subsequent recoveries, if any, are credited to the allowance. We estimate the allowance balance required using past loan loss experience, the nature and volume of the portfolio, information about specific borrower situations and estimated collateral values, economic conditions and other factors. Allocations of the allowance may be made for specific loans, but the entire allowance is available for any loan that, in our judgment, should be charged-off. A loan is considered to be impaired when, based on current information and events, it is probable we will be unable to collect the scheduled payments of principal and interest when due according to the contractual terms of the loan agreement. Factors considered in determining impairment include payment status, collateral value and the probability of collecting scheduled principal and interest payments when due. Loans that experience insignificant payment delays and payment shortfalls generally are not classified as impaired. We determine the significance of payment delays and payment shortfalls on a case-by-case basis, taking into consideration all of the circumstances surrounding the loan and the borrower, including the length of delay, the reasons for delay, the borrower’s prior payment record and the amount of the shortfall in relation to the principal and interest owed. Impairment is measured on a loan-by-loan basis for commercial and construction loans by either the present value of expected future cash flows discounted at the loan’s effective interest rate, the loan’s obtainable market price or the fair value of collateral if the loan is collateral dependent. Large groups of smaller balance homogeneous loans are collectively evaluated for impairment.
Troubled Debt Restructurings	Troubled Debt Restructurings: A loan is accounted for as a troubled debt restructuring if we, for economic or legal reasons, grant a concession to a borrower considered to be experiencing financial difficulties that we would not otherwise consider. A troubled debt restructuring may involve the receipt of assets from the debtor in partial or full satisfaction of the loan, or a modification of terms such as a reduction of the stated interest rate or balance of the loan, a reduction of accrued interest, an extension of the maturity date or renewal of the loan at a stated interest rate lower than the current market rate for a new loan with similar risk, or some combination of these concessions. Troubled debt restructurings can be in either accrual or nonaccrual status. Nonaccrual troubled debt restructurings are included in nonperforming loans. Accruing troubled debt restructurings are generally excluded from nonperforming loans as it is considered probable that all contractual principal and interest due under the restructured terms will be collected. In accordance with current accounting guidance, loans modified as troubled debt restructurings are, by definition, considered to be impaired loans. Impairment for these loans is measured on a loan-by-loan basis similar to other impaired loans as described above under “Allowance for Loan Losses.” Certain loans modified as troubled debt restructurings may have been previously measured for impairment under a general allowance methodology (i.e., pooling), thus at the time the loan is modified as a troubled debt restructuring the allowance will be impacted by the difference between the results of these two measurement methodologies. Loans modified as troubled debt restructurings that subsequently default are factored into the determination of the allowance in the same manner as other defaulted loans.
Derivatives	Derivatives: Derivative financial instruments are recognized as assets or liabilities at fair value. The accounting for changes in the fair value of derivatives depends on the use of the derivatives and whether the derivatives qualify for hedge accounting. Used as part of our asset and liability management to help manage interest rate risk, our derivatives have historically consisted of interest rate swap agreements that qualified for hedge accounting. In February 2012, we entered into an interest rate swap agreement that qualifies for hedge accounting. However, in June 2011, we simultaneously purchased and sold an interest rate cap, a structure commonly referred to as a “cap corridor”, which does not qualify for hedge accounting. The current outstanding interest rate swap and cap corridor are discussed in more detail in Note 9. We do not use derivatives for trading purposes. Changes in the fair value of derivatives that are designated, for accounting purposes, as a hedge of the variability of cash flows to be received on various loans and are effective are reported in other comprehensive income. They are later reclassified into earnings in the same periods during which the hedged transaction affects earnings and are included in the line item in which the hedged cash flows are recorded. If hedge accounting does not apply, changes in the fair value of derivatives are recognized immediately in current earnings as interest income or expense. If designated as a hedge, we formally document the relationship between derivatives as hedged items, as well as the risk-management objective and the strategy for undertaking hedge transactions. This documentation includes linking cash flow hedges to specific assets and liabilities on the balance sheet. If designated as a hedge, we also formally assess, both at the hedge’s inception and on an ongoing basis, whether the derivative instruments that are used are highly effective in offsetting changes in cash flows of the hedged items. Ineffective hedge gains and losses are recognized immediately in current earnings as noninterest income or expense. We discontinue hedge accounting when we determine the derivative is no longer effective in offsetting changes in the cash flows of the hedged item, the derivative is settled or terminates, or treatment of the derivative as a hedge is no longer appropriate or intended.
Adoption of New Accounting Standards	Adoption of New Accounting Standards: In February 2013, the FASB issued ASU 2013-02, Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income, which is intended to improve the reporting of reclassifications out of accumulated other comprehensive income. The ASU requires an entity to report, either on the face of the income statement or in the notes to the financial statements, the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in the income statement. We adopted this ASU in the first quarter of 2013.

Disclosure of accounting policy for its derivative instruments and hedging activities.

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Disclosure of accounting policy for computing basic and diluted earnings or loss per share for each class of common stock and participating security. Addresses all significant policy factors, including any antidilutive items that have been excluded from the computation and takes into account stock dividends, splits and reverse splits that occur after the balance sheet date of the latest reporting period but before the issuance of the financial statements.

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- Definition

Disclosure of accounting policy for those finance, loan and lease receivables classified as held for investment. This disclosure may include (1) the basis at which such receivables are carried in the entity's statements of financial position (2) how the level of the allowance for loan and lease losses is determined (3) when impairments, charge-offs or recoveries are recognized for such receivables (4) the treatment of commitment and other fees and loan origination costs (including, if applicable, how the entity accounts for fees and costs associated with credit cards that are either purchased or originated) (5) the treatment of any premiums or discounts or unearned income (6) the entity's income recognition policies for such receivables, including those that are impaired, past due or placed on nonaccrual status (for impaired loans, the policy for recognizing interest income on such loans, including how cash receipts are recorded) and (7) the treatment of foreclosures or repossessions.

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