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Note 3 - Loans and Allowance for Credit Losses - Activity for Troubled Debt Restructurings (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022		Mar. 31, 2021
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member]
Beginning Balance	$ 4,973,000		$ 6,414,000
Charge-Offs	0		0
Payments	(212,000)		(2,676,000)
Transfers to ORE	0		0
Net Additions/Deletions	(1,932,000)		22,000
Ending Balance	2,829,000		3,760,000
Commercial Portfolio Segment [Member] | Vacant Land and Land Development and Residential Construction Loan [Member]
Beginning Balance	0		0
Charge-Offs	0		0
Payments	0		0
Transfers to ORE	0		0
Net Additions/Deletions	0		0
Ending Balance	0		0
Commercial Portfolio Segment [Member] | Real Estate Owner Occupied Loan [Member]
Beginning Balance	10,435,000		14,797,000
Charge-Offs	0		0
Payments	(9,677,000)		(910,000)
Transfers to ORE	0		0
Net Additions/Deletions	(669,000)		0
Ending Balance	89,000		13,887,000
Commercial Portfolio Segment [Member] | Real Estate Non Owner Occupied Loan [Member]
Beginning Balance	146,000		480,000
Charge-Offs	0		0
Payments	(3,000)		(9,000)
Transfers to ORE	0		0
Net Additions/Deletions	0		0
Ending Balance	143,000		471,000
Commercial Portfolio Segment [Member] | Real Estate Multi Family and Residential Rental Loan [Member]
Beginning Balance	91,000		0
Charge-Offs	0		0
Payments	(1,000)		0
Transfers to ORE	0		0
Net Additions/Deletions	0		0
Ending Balance	90,000		0
Retail Portfolio Segment [Member] | One to Four Family Mortgages [Member]
Beginning Balance	627,000		806,000
Charge-Offs	0		0
Payments	(78,000)		(22,000)
Transfers to ORE	0		0
Net Additions/Deletions	1,797,000	[1]	36,000
Ending Balance	2,346,000		820,000
Retail Portfolio Segment [Member] | Other Consumer Loans [Member]
Beginning Balance	1,202,000		1,146,000
Charge-Offs	0		0
Payments	(2,000)		(118,000)
Transfers to ORE	0		0
Net Additions/Deletions	(1,187,000)	[1]	71,000
Ending Balance	$ 13,000		$ 1,099,000
[1] Includes $1.2 million in the transfer of home equity lines of credit from other consumer loans to 1-4 family mortgages in association with the adoption of the CECL methodology effective January 1, 2022.

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

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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

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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

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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

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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 https://asc.fasb.org/extlink&oid=123577603&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 https://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

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- Details

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- Details

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- Details

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- Details

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