The number of shares that each share of the acquiree is converted into shares of the acquirer.

No definition available.

Name:	mbwm_BusinessAcquisitionConversionOfStockPerShare
Namespace Prefix:	mbwm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

For receivables acquired in a business combination, including certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectible
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For receivables acquired in a business combination, including certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the future interest not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectibleInterest
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of expense to write-down the amortized cost basis of loans, before acquisition, to fair value. Expressed as a component of nonaccretable difference.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansPreacquisitionChargeoffs
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the unaccreted discount.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationAcquiredReceivablesUnaccredtedDiscount
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This represents the aggregate in merger related expenses recorded by Mercantile Bank and Firstbank during 2014 and 2013.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationAggregateMergerRelatedExpensesRecordedByAcquirerAndAcquiree
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of banking branches acquired in a business combination.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationBranchesAcquired
Namespace Prefix:	mbwm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to net deferred tax asset account in connection with a business combination for which the initial accounting was incomplete.

+ References

No definition available.

+ Details

Name:	mbwm_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNetDeferredTaxAsset
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	mbwm_Note2BusinessCombinationDetailsLineItems
Namespace Prefix:	mbwm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)(4)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the gross contractual amounts receivable, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(2)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationAcquiredReceivablesGrossContractualAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 23

-URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e1043-128460

+ Details

Name:	us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of assets acquired at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e961-128460

+ Details

Name:	us-gaap_GoodwillPurchaseAccountingAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=mbwm_FirstbankCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Note 2 - Business Combination (Details) (USD $)	12 Months Ended			0 Months Ended		6 Months Ended	24 Months Ended
Note 2 - Business Combination (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jun. 01, 2014	Jun. 30, 2014	Dec. 31, 2014
Note 2 - Business Combination (Details) [Line Items]
Goodwill	$ 49,473,000us-gaap_Goodwill	$ 0us-gaap_Goodwill		$ 49,473,000us-gaap_Goodwill			$ 49,473,000us-gaap_Goodwill
Business Combination, Acquisition Related Costs	5,447,000	1,246,000	0
Firstbank Corporation [Member]
Note 2 - Business Combination (Details) [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					1,452,880,000
Business Combination, Branches Acquired					46
Business Acquisition, Conversion of Stock Per Share (in Shares)					1
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares (in Shares)					8,087,272	8,087,272
Goodwill					49,473,000
Goodwill, Purchase Accounting Adjustments				(1,400,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment Net Deferred Tax Asset				1,400,000
Business Combination, Acquisition Related Costs	5,400,000	1,200,000
Business Combination, Aggregate Merger Related Expenses Recorded by Acquirer and Acquiree							12,200,000
Business Combination, Acquired Receivables and Certain Loans, Estimated Uncollectible Cash Flows					10,400,000
Business Combination, Acquired Receivables and Certain Loans, Pre-acquisition Charge-offs					2,800,000
Business Combination, Acquired Receivables and Certain Loans, Estimated Uncollectible Interest					3,900,000
Business Combination, Acquired Receivables, Gross Contractual Amount					926,400,000
Business Combination, Acquired Receivables, Unaccredted Discount					$ 8,100,000