Note 3 - Loans and Allowance for Loan Losses - Activity for Troubled Debt Restructurings (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member]
Beginning Balance	$ 12,204,000	$ 15,610,000	$ 8,587,000	$ 14,138,000
Charge-Offs	0	0	0	0
Payments	(2,143,000)	(3,148,000)	(4,975,000)	(4,814,000)
Transfers to ORE	0	0	0	0
Net Additions/Deletions	6,000	11,672,000	6,455,000	14,810,000
Ending Balance	10,067,000	24,134,000	10,067,000	24,134,000
Commercial Portfolio Segment [Member] \| Vacant Land and Land Development and Residential Construction Loan [Member]
Beginning Balance	82,000	0	85,000	0
Charge-Offs	0	0	0	0
Payments	(2,000)	0	(5,000)	0
Transfers to ORE	0	0	0	0
Net Additions/Deletions	0	0	0	0
Ending Balance	80,000	0	80,000	0
Commercial Portfolio Segment [Member] \| Real Estate Owner Occupied Loan [Member]
Beginning Balance	3,811,000	3,695,000	1,145,000	3,100,000
Charge-Offs	0	0	0	0
Payments	(20,000)	(2,377,000)	(1,008,000)	(2,425,000)
Transfers to ORE	0	0	0	(97,000)
Net Additions/Deletions	0	1,180,000	3,654,000	1,920,000
Ending Balance	3,791,000	2,498,000	3,791,000	2,498,000
Commercial Portfolio Segment [Member] \| Real Estate Non Owner Occupied Loan [Member]
Beginning Balance	174,000	189,000	178,000	210,000
Charge-Offs	0	0	0	0
Payments	(4,000)	(4,000)	(8,000)	(25,000)
Transfers to ORE	0	0	0	0
Net Additions/Deletions	0	0	0	0
Ending Balance	170,000	185,000	170,000	185,000
Commercial Portfolio Segment [Member] \| Real Estate Multi Family and Residential Rental Loan [Member]
Beginning Balance	3,000	21,000	7,000	24,000
Charge-Offs	0	0	0	0
Payments	(2,000)	(4,000)	(6,000)	(7,000)
Transfers to ORE	0	0	0	0
Net Additions/Deletions	0	0	0	0
Ending Balance	1,000	17,000	1,000	17,000
Retail Portfolio Segment [Member] \| Home Equity and Other [Member]
Beginning Balance	1,452,000	1,444,000	1,415,000	1,402,000
Charge-Offs	0	(18,000)	0	(18,000)
Payments	(175,000)	(62,000)	(203,000)	(90,000)
Transfers to ORE	0	0	0	0
Net Additions/Deletions	438,000	102,000	503,000	172,000
Ending Balance	1,715,000	1,466,000	1,715,000	1,466,000
Retail Portfolio Segment [Member] \| One to Four Family Mortgages [Member]
Beginning Balance	715,000	600,000	724,000	578,000
Charge-Offs	0	0	0	0
Payments	(20,000)	(14,000)	(29,000)	(24,000)
Transfers to ORE	0	0	0	0
Net Additions/Deletions	20,000	120,000	20,000	152,000
Ending Balance	$ 715,000	$ 706,000	$ 715,000	$ 706,000

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

No definition available.

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

- 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

- 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_
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Balance Type:	credit
Period Type:	duration

- 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

- 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

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