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Note 18 - Variable Interest Entities - Business Trusts (Details) - USD ($) $ in Millions		12 Months Ended
	Dec. 15, 2021	Dec. 31, 2024	Dec. 31, 2023	Jan. 26, 2016
Preferred securities outstanding		$ 48.3	$ 47.6
Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration]	Secured Overnight Financing Rate (SOFR) [Member]	Secured Overnight Financing Rate (SOFR) [Member]
Mercantile Bank Capital Trust I [Member]
Preferred securities outstanding		$ 21.0		$ 32.0
Interest rate margin		2.18%
Firstbank Capital Trust I [Member]
Preferred securities outstanding		$ 10.0
Interest rate margin		1.99%
Firstbank Capital Trust II [Member]
Preferred securities outstanding		$ 10.0
Interest rate margin		1.27%
Firstbank Capital Trust III [Member]
Preferred securities outstanding		$ 7.5
Interest rate margin		1.35%
Firstbank Capital Trust IV [Member]
Preferred securities outstanding		$ 7.5
Interest rate margin		1.35%

X

- Definition

Trust preferred securities possesses characteristics of both equity and debt issues and are generally issued by bank holding companies.

+ References

No definition available.

+ Details

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

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Indicates type of variable interest rate on debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477734/942-470-50-3

+ Details

Name:	us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

Name:	us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIIIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIVMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: