Note 8 - Commitments and Off-balance Sheet Risk (Details) - USD ($)	9 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Note 8 - Commitments and Off-balance Sheet Risk (Details) [Line Items]
Reserve or Liability Balance for Financial Instruments with Off-balance Sheet Risk	$ 0	$ 0
Interest Rate Swap [Member]
Note 8 - Commitments and Off-balance Sheet Risk (Details) [Line Items]
Derivative, Notional Amount	14,800,000
Derivative, Fair Value, Net	$ (2,600,000)
Minimum [Member]
Note 8 - Commitments and Off-balance Sheet Risk (Details) [Line Items]
Fees Accreted Into Income During Interest Rate Swap Agreements, Term	4 years
Maximum [Member]
Note 8 - Commitments and Off-balance Sheet Risk (Details) [Line Items]
Fees Accreted Into Income During Interest Rate Swap Agreements, Term	15 years

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of time between issuance and maturity of interest rate swap agreements, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	mbwm_FeesAccretedIntoIncomeDuringInterestRateSwapAgreementsTerm
Namespace Prefix:	mbwm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	mbwm_Note8CommitmentsandOffbalanceSheetRiskDetailsLineItems
Namespace Prefix:	mbwm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying amount for the reserve or liability for financial instruments with off-balance sheet risk.

+ References

No definition available.

+ Details

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

- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13495-108611

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: