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Note 3 - Loans and Allowance for Loan Losses - Activity for Troubled Debt Restructurings (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member]
Beginning Balance	$ 8,587,000	$ 14,138,000
Charge-Offs	0	0
Payments	(2,832,000)	(1,666,000)
Transfers to ORE	0	0
Net Additions/Deletions	6,449,000	3,138,000
Ending Balance	12,204,000	15,610,000
Commercial Portfolio Segment [Member] | Vacant Land and Land Development and Residential Construction Loan [Member]
Beginning Balance	85,000	0
Charge-Offs	0	0
Payments	(3,000)	0
Transfers to ORE	0	0
Net Additions/Deletions	0	0
Ending Balance	82,000	0
Commercial Portfolio Segment [Member] | Real Estate Owner Occupied Loan [Member]
Beginning Balance	1,145,000	3,100,000
Charge-Offs	0	0
Payments	(988,000)	(48,000)
Transfers to ORE	0	(97,000)
Net Additions/Deletions	3,654,000	740,000
Ending Balance	3,811,000	3,695,000
Commercial Portfolio Segment [Member] | Real Estate Non Owner Occupied Loan [Member]
Beginning Balance	178,000	210,000
Charge-Offs	0	0
Payments	(4,000)	(21,000)
Transfers to ORE	0	0
Net Additions/Deletions	0	0
Ending Balance	174,000	189,000
Commercial Portfolio Segment [Member] | Real Estate Multi Family and Residential Rental Loan [Member]
Beginning Balance	7,000	24,000
Charge-Offs	0	0
Payments	(4,000)	(3,000)
Transfers to ORE	0	0
Net Additions/Deletions	0	0
Ending Balance	3,000	21,000
Retail Portfolio Segment [Member] | Home Equity and Other [Member]
Beginning Balance	1,415,000	1,402,000
Charge-Offs	0	0
Payments	(29,000)	(28,000)
Transfers to ORE	0	0
Net Additions/Deletions	66,000	70,000
Ending Balance	1,452,000	1,444,000
Retail Portfolio Segment [Member] | One to Four Family Mortgages [Member]
Beginning Balance	724,000	578,000
Charge-Offs	0	0
Payments	(9,000)	(11,000)
Transfers to ORE	0	0
Net Additions/Deletions	0	32,000
Ending Balance	$ 715,000	$ 599,000

X

- Definition

A financing receivable which had its terms modified during the period and is now considered a troubled restructuring.

+ References

No definition available.

+ Details

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

X

- Definition

Reflects the amount of the charge-off related to a troubled debt restructuring.

+ References

No definition available.

+ Details

Name:	mbwm_FinancingReceivableModifiedAsTroubledDebtRestructuringsChargeOffs
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The cash inflow associated with a financing receivable classified as a troubled debt restructuring.

+ References

No definition available.

+ Details

Name:	mbwm_FinancingReceivableModifiedAsTroubledDebtRestructuringsPayments
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The transfer of a financing receivable classified as a troubled debt restructuring to other real estate.

+ References

No definition available.

+ Details

Name:	mbwm_FinancingReceivableModifiedAsTroubledDebtRestructuringsTransferToOtherRealEstate
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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- Details

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Namespace Prefix:
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Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Period Type:

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- Details

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Namespace Prefix:
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