Note 3 - Loans and Allowance for Loan Losses - Loans Modified as Troubled Debt Restructurings (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Number of Contracts	12	9	34	28
Pre- Modification Recorded Principal Balance	$ 784,000	$ 328,000	$ 11,074,000	$ 16,640,000
Post- Modification Recorded Principal Balance	$ 784,000	$ 327,000	$ 10,478,000	$ 16,938,000
Commercial Portfolio Segment [Member]
Number of Contracts	5	3	17	10
Pre- Modification Recorded Principal Balance	$ 634,000	$ 217,000	$ 10,564,000	$ 16,213,000
Post- Modification Recorded Principal Balance	$ 634,000	$ 217,000	$ 9,968,000	$ 16,510,000
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member]
Number of Contracts	2	1	8	7
Pre- Modification Recorded Principal Balance	$ 7,000	$ 28,000	$ 6,485,000	$ 14,457,000
Post- Modification Recorded Principal Balance	$ 7,000	$ 28,000	$ 6,491,000	$ 14,754,000
Commercial Portfolio Segment [Member] \| Vacant Land and Land Development and Residential Construction Loan [Member]
Number of Contracts	0	1	0	1
Pre- Modification Recorded Principal Balance	$ 0	$ 87,000	$ 0	$ 87,000
Post- Modification Recorded Principal Balance	$ 0	$ 87,000	$ 0	$ 87,000
Commercial Portfolio Segment [Member] \| Real Estate Owner Occupied Loan [Member]
Number of Contracts	1	1	7	2
Pre- Modification Recorded Principal Balance	$ 56,000	$ 102,000	$ 3,508,000	$ 1,669,000
Post- Modification Recorded Principal Balance	$ 56,000	$ 102,000	$ 2,906,000	$ 1,669,000
Commercial Portfolio Segment [Member] \| Real Estate Non Owner Occupied Loan [Member]
Number of Contracts	2	0	2	0
Pre- Modification Recorded Principal Balance	$ 571,000	$ 0	$ 571,000	$ 0
Post- Modification Recorded Principal Balance	$ 571,000	$ 0	$ 571,000	$ 0
Commercial Portfolio Segment [Member] \| Real Estate Multi Family and Residential Rental Loan [Member]
Number of Contracts	0	0	0	0
Pre- Modification Recorded Principal Balance	$ 0	$ 0	$ 0	$ 0
Post- Modification Recorded Principal Balance	$ 0	$ 0	$ 0	$ 0
Retail Portfolio Segment [Member]
Number of Contracts	7	6	17	18
Pre- Modification Recorded Principal Balance	$ 150,000	$ 111,000	$ 510,000	$ 427,000
Post- Modification Recorded Principal Balance	$ 150,000	$ 110,000	$ 510,000	$ 428,000
Retail Portfolio Segment [Member] \| Home Equity and Other [Member]
Number of Contracts	5	5	14	13
Pre- Modification Recorded Principal Balance	$ 82,000	$ 62,000	$ 422,000	$ 225,000
Post- Modification Recorded Principal Balance	$ 83,000	$ 61,000	$ 423,000	$ 225,000
Retail Portfolio Segment [Member] \| One to Four Family Mortgages [Member]
Number of Contracts	2	1	3	5
Pre- Modification Recorded Principal Balance	$ 68,000	$ 49,000	$ 88,000	$ 202,000
Post- Modification Recorded Principal Balance	$ 67,000	$ 49,000	$ 87,000	$ 203,000

Number of financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_VacantLandAndLandDevelopmentAndResidentialConstructionLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_RealEstateOwnerOccupiedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_RealEstateNonOwnerOccupiedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_RealEstateMultiFamilyAndResidentialRentalLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=mbwm_RetailPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_HomeEquityAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_OneToFourFamilyMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: