Note 3 - Loans and Allowance for Credit Losses (Details Textual) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Financing Receivable, Nonaccrual, Interest Income	$ 1.0	$ 0.3	$ 0.2
Financing Receivable, Revolving, Converted to Term Loan During Period	0.7	9.1
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease) From Forecast Changes	(0.3)	(2.2)
Financing Receivable, Individually Evaluated for Credit Loss	7.9	7.4
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 4.0	$ 2.2
Commercial Portfolio Segment [Member]
Financing Receivables, Prepayment Speed Assumed	2.00%	2.00%
Retail Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)	$ (2.3)
Retail Portfolio Segment [Member] \| One to Four Family Mortgages [Member]
Financing Receivables, Prepayment Speed Assumed	9.50%	7.80%
Retail Portfolio Segment [Member] \| Other Consumer Loans [Member]
Financing Receivables, Prepayment Speed Assumed	9.50%	7.80%

Amount of increase (decrease) in allowance for credit loss on financing receivable due economic forecast changes.

No definition available.

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Period Type:	duration

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

No definition available.

+ Details

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

- Definition

Percentage of prepayment speed assumed under financing receivables.

+ References

No definition available.

+ Details

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Namespace Prefix:	mbwm_
Data Type:	dtr-types:percentItemType
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- Definition

Amount of increase (decrease) in allowance for credit loss on financing receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable individually evaluated for credit loss. Excludes financing receivable collectively evaluated for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.5.a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11C

+ Details

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

- Definition

Amount of interest income on financing receivable on nonaccrual status.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in term loan from conversion of revolving financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6A

+ Details

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Period Type:	duration

- Details

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- Details

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- Details

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- Details

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