Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Accounting, Policy [Policy Text Block]	Basis of Presentation : The unaudited financial statements for the nine months ended September 30, 2017 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, 2017 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, 2016 . We have five separate business trusts that were formed to issue trust preferred securities. Subordinated debentures were issued to the trusts in return for the proceeds raised from the issuance of the trust preferred securities. The trusts are not consolidated, but instead we report the subordinated debentures issued to the trusts as a liability.
Earnings Per Share, Policy [Policy Text Block]	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 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 220,000 unvested restricted shares were included in determining both basic and diluted earnings per share for the three and nine months ended September 30, 2017 . In addition, stock options for approximately 28,000 shares of common stock were included in determining diluted earnings per share for the three and nine months ended September 30, 2017 . Stock options for approximately 7,000 shares of common stock were antidilutive and not included in determining diluted earnings per share for the three and nine months ended September 30, 2017 . Approximately 148 ,000 unvested restricted shares were included in determining both basic and diluted earnings per share for the three and nine months ended September 30, 2016 . In addition, stock options for approximately 79,000 shares of common stock were included in determining diluted earnings per share for the three and nine months ended September 30, 2016 . Stock options for approximately 4,000 shares of common stock were antidilutive and not included in determining diluted earnings per share for the three and nine months ended September 30, 2016 .
Marketable Securities, Policy [Policy Text Block]	Securities : Debt securities classified as held to maturity are carried at amortized cost when management has the positive intent and ability to hold them to maturity. Debt securities are classified as available for sale when they might be sold prior to maturity. Equity securities with readily determinable fair values are classified as available for sale. Securities available for sale are carried at fair value, with unrealized holding gains and losses reported in other comprehensive income, net of tax. Federal Home Loan Bank stock is carried at cost. Interest income includes amortization of purchase premiums and accretion of discounts. Premiums and discounts on securities are amortized or accreted on the level-yield method without anticipating prepayments, except for mortgage-backed securities where prepayments are anticipated. Gains and losses on sales are recorded on the trade date and determined using the specific identification method. Declines in the fair value of debt securities below their amortized cost that are other than temporary (“ OTTI”) are reflected in earnings or other comprehensive income, as appropriate. For those debt securities whose fair value is less than their amortized cost, we consider our intent to sell the security, whether it is more likely than not that we will be required to sell the security before recovery and whether we expect to recover the entire amortized cost of the security based on our assessment of the issuer’s financial condition. In analyzing an issuer’s financial condition, we consider whether the securities are issued by the federal government or its agencies, and whether downgrades by bond rating agencies have occurred. If either of the criteria regarding intent or requirement to sell is met, the entire difference between amortized cost and fair value is recognized as impairment through earnings. For debt securities that do not meet the aforementioned criteria, the amount of impairment is split into two components as follows: 1 ) OTTI related to credit loss, which must be recognized in the income statement, and 2 ) OTTI related to other factors, such as liquidity conditions in the market or changes in market interest rates, which is recognized in other comprehensive income. The credit loss is defined as the difference between the present value of the cash flows expected to be collected and the amortized cost.
Finance, Loan and Lease Receivables, Held-for-investment, Policy [Policy Text Block]	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 commerci al 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.
Finance, Loan and Lease Receivables, Held-for-sale, Policy [Policy Text Block]	Loans Held for Sale : Mortgage loans originated and intended for sale in the secondary market are carried at the lower of aggregate cost or market, as determined by outstanding commitments from investors. Net unrealized losses, if any, are recorded as a valuation allowance and charged to earnings. As of September 30, 2017 and December 31, 2016, we determined that the fair value of our mortgage loans held for sale approximated the recorded cost of $4.1 million and $1. 0 million, respectively. Loans held for sale are reported as part of our total loans on the balance sheet. Mortgage loans held for sale are generally sold with servicing rights retained. Gains and losses on sales of mortgage loans are based on the difference between the selling price and the carrying value of the related loan sold, which is reduced by the cost allocated to the servicing right. We generally lock in the sale price to the purchaser of the loan at the same time we make a rate commitment to the borrower. These mortgage banking activities are not designated as hedges and are carried at fair value. The net gain or loss on mortgage banking derivatives is included in the gain on sale of loans. Mortgage loans serviced for others totaled approximately $611 million as of September 30, 2017 .
Loans and Leases Receivable, Mortgage Banking Activities, Policy [Policy Text Block]	Mortgage Banking Activities : Mortgage loan servicing rights are recognized as assets based on the allocated value of retained servicing rights on mortgage loans sold. Mortgage loan servicing rights are carried at the lower of amortized cost or fair value and are expensed in proportion to, and over the period of, estimated net servicing revenues. Impairment is evaluated based on the fair value of the rights using groupings of the underlying mortgage loans as to interest rates. Any impairment of a grouping is reported as a valuation allowance. Servicing fee income is rec orded for fees earned for servicing mortgage loans. The fees are based on a contractual percentage of the outstanding principal or a fixed amount per loan and are recorded as income when earned. Amortization of mortgage loan servicing rights is netted against mortgage loan servicing income and recorded in mortgage banking activities in the income statement.
Loans and Leases Receivable, Troubled Debt Restructuring Policy [Policy Text Block]	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 ac cordance 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 below 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.
Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block]	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 impa ired 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.
Derivatives, Policy [Policy Text Block]	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 generally 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. The current outstanding interest rate swap is 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 assets and liabilities 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 the derivatives and hedged items, as well as the risk-management objective and the strategy for undert aking 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.
Goodwill and Intangible Assets, Policy [Policy Text Block]	Goodwill and Core Deposit Intangible : Goodwill results from business acquisitions and represents the excess of the purchase price over the fair value of acquired tangible assets and liabilities and identifiable intangible assets. Goodwill is assessed at least annually for impairment and any such impairment is recognized in the period identified. A more frequent assessment is performed should events or changes in circumstances indicate the carrying value of the goodwill may not be recoverable. We may elect to perform a qualitative assessment for the annual impairment test. If the qualitative assessment indicates it is more likely than not that the fair value of a reporting unit is less than its carrying amount, or if we elect not to perform a qualitative assessment, then we would be required to perform a quantitative test for goodwill impairment. The quantitative test is a two -step process consisting of comparing the carrying value of the reporting unit to an estimate of its fair value. If the estimated fair value of the reporting unit is less than the carrying value, goodwill is impaired and is written down to its estimated fair value. In 2015 and 2016, we elected to perform a qualitative assessment for our annual impairment test and concluded it is more likely than not our fair value was greater than its carrying amount; therefore, no further testing was required. The core deposit intangible that arose from the Firstbank Corporation acquisition was initially measured at fair value and is being amortized into n oninterest expense over a ten -year period using the sum-of-the-years-digits methodology.
New Accounting Pronouncements, Policy [Policy Text Block]	Adoption of New Accounting Standards: In May 2014, the FASB issued ASU 2014 - 09, Revenue from Contracts with Customers . This ASU establishes a comprehensive revenue recognition standard for virtually all industries under U.S. GAAP, including those that previously followed industry-specific guidance such as the real estate, construction and software industries. The revenue standard’s core principle is built on the contract between a vendor and a customer for the provision of goods and services. It attempts to depict the exchange of rights and obligations between the parties in the pattern of revenue recognition based on the consideration to which the vendor is entitled. To accomplish this objective, the standard requires five basic steps: i) identify the contract with the customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies a performance obligation. This ASU was originally effective for annual and interim periods beginning after December 15, 2016, with two transition methods available – full retrospective and cumulative effect approach. In August 2015, the FASB issued ASU 2015 - 14, Revenue from Contracts with Customers – Deferral of Effective Date , which delays the implementation of this guidance by one year. Since the guidance does not apply to revenue associated with financial instruments, including loans and securities that are accounted for under other GAAP, the new guidance will not have an impact on interest income. We are completing an overall assessment of revenue streams and review of related contracts potentially affected by the ASU. In addition, we are evaluating the ASU’s expanded disclosure requirements. We plan to adopt this ASU on January 1, 2018 utilizing the modified retrospective approach with a cumulative effect adjustment to opening retained earnings, if such adjustment is applicable. In January 2016, the FASB issued ASU 2016 - 1, Recognition and Measurement of Financial Assets and Financial Liabilities . This ASU requires an entity to (i) measure equity investments at fair value through net income, with certain exceptions; (ii) present in OCI the changes in instrument-specific credit risk for financial liabilities measured using the fair value option; (iii) present financial assets and financial liabilities by measurement category and form of financial asset; (iv) calculate the fair value of financial instruments for disclosure purposes based on an exit price; and (v) assess a valuation allowance on deferred tax assets related to unrealized losses on available for sale debt securities in combination with other deferred tax assets. This ASU provides an election to subsequently measure certain nonmarketable equity investments at cost less any impairment and adjusted for certain observable price changes. This ASU also requires a qualitative impairment assessment of such equity investments and amends certain fair value disclosure requirements. The amendments are effective for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017, and are not expected to have a material effect on our financial position or results of operations when adopted. In February 2016, the FASB issued ASU 2016 - 02, Leases . This ASU establishes a right-of-use (“ROU”) model that requires a lessee to record a ROU asset and a lease liability on the balance sheet for all leases with terms longer than 12 months. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. The ASU is effective for annual and interim periods beginning after December 15, 2018. A modified retrospective transition approach is required for lessees for capital and operating leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. Adoption of this ASU is not expected to have a material effect on our financial position or results of operations. In March 2016, the FASB issued ASU 2016 - 09, Compensation – Stock Compensation: Improvements to Employee Share-Based Payment Accounting . This ASU requires that, prospectively, all tax effects related to share-based payments be made through the income statement at the time of settlement as opposed to excess tax benefits being recognized in additional paid-in capital under the current guidance. The ASU also removes the requirement to delay recognition of a tax benefit until it reduces current taxes payable. This change is required to be applied on a modified retrospective basis, with a cumulative-effect adjustment to opening retained earnings. Additionally, all tax related cash flows resulting from share-based payments are to be reported as operating activities on the statement of cash flows, a change from the current requirement to present tax benefits as an inflow from financing activities and an outflow from operating activities. Finally, entities will be allowed to withhold an amount up to the employees’ maximum individual tax rate (as opposed to the minimum statutory tax rate) in the relevant jurisdiction without resulting in liability classification of the award. The change in withholding requirements will be applied on a modified retrospective approach. This standard will be effective for annual and interim periods beginning after December 15, 2016. Adoption of this ASU did not have a material effect on our financial position or results of operations. In June 2016, the FASB issued ASU No. 2016 - 13, Measurement of Credit Losses on Financial Instruments . This ASU significantly changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The standard will replace the current “incurred loss” approach with an “expected loss” model. The new model, referred to as the current expected credit loss (“CECL”) model, will apply to: (i) financial assets subject to credit losses and measured at amortized cost, and (ii) certain off-balance sheet credit exposures. This includes, but is not limited to, loans, leases, held-to-maturity securities, loan commitments and financial guarantees. The ASU also simplifies the accounting model for purchased credit-impaired debt securities and loans, and expands the disclosure requirements regarding an entity’s assumptions, models, and methods for estimating the allowance for loan and lease losses. In addition, entities will need to disclose the amortized cost balance for each class of financial asset by credit quality indicator, disaggregated by the year of origination. This ASU is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted for interim and annual reporting periods beginning after December 15, 2018. Entities will apply the standard’s provisions as a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective (i.e., modified retrospective approach). We are currently evaluating the provisions of this ASU to determine the potential impact the new standard will have on our consolidated financial statements. In August 2016, the FASB issued ASU No. 2016 - 15, Statement of Cash Flows (Topic 230 ): Classification of Certain Cash Receipts and Cash Payments . This ASU will make eight targeted changes to how cash receipts and cash payments are presented and classified in the statement of cash flows and is effective for fiscal years beginning after December 15, 2017. The new standard will require adoption on a retrospective basis unless it is impractical to apply, in which case it would be required to apply the amendments prospectively as of the earliest date practicable. We are currently evaluating the provisions of this ASU to determine the potential impact the new standard will have on our consolidated financial statements. In J anuary 2017, the FASB issued ASU No. 2017 - 04, Goodwill and Other (Topic 350 ): Simplifying the Test for Goodwill Impairment . This ASU simplifies the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. Under this ASU, an entity should perform the Step 1 annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying value. If the carrying amount exceeds the fair value, an entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value, not to exceed the total amount of goodwill allocated to that reporting unit. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. This ASU is effective January 1, 2020 and early adoption is permitted. The ASU should be applied prospectively. Adoption of this ASU is not expected to have a material effect on our financial position or results of operations. In March 2017, the FASB issued ASU No. 2017 - 08, Premium Amortization on Purchased Callable Debt Securities . This ASU requires the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. Previously, entities were allowed to amortize to contractual maturity or to call date. This ASU is effective for annual reporting periods beginning after December 15, 2018, and early adoption is permitted. We are currently evaluating the provisions of this ASU to determine the potential impact the new standard will have on our consolidated financial statements.