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Note 3 - Loans and Allowance for Loan Losses (Details Textual) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Loans and Leases Receivable, Net of Deferred Income, Total	$ 2,901,543,000		$ 2,856,667,000
Loans and Leases Receivable Period Increase (Decrease)	$ 44,900,000
Loans and Leases Receivable, Percentage Period Increase (Decrease)	1.60%
Impaired Financing Receivable, Interest Income, Accrual Method, Total	$ 300,000	$ 300,000
Loans and Leases Receivable, Nonaccrual Loans, Interest Income Recognized	0	0
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	$ 100,000	$ 100,000
Minimum Number of Days Past Due at Period End For Loans Modified as Troubled Debt Restructurings (Day)	30 days

X

- Definition

Amount of interest that was recorded on nonaccrual loans that were paid off during the period.

+ References

No definition available.

+ Details

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

X

- Definition

The percentage of increase or (decrease) during the period in loans and leases receivable during the period.

+ References

No definition available.

+ Details

Name:	mbwm_LoansAndLeasesReceivablePercentagePeriodIncreaseDecrease
Namespace Prefix:	mbwm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The monetary increase or (decrease) in loans and leases receivable during the period.

+ References

No definition available.

+ Details

Name:	mbwm_LoansAndLeasesReceivablePeriodIncreaseDecrease
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the minimum number of days past due at period end for loans modified as troubled debt restructurings.

+ References

No definition available.

+ Details

Name:	mbwm_MinimumNumberOfDaysPastDueAtPeriodEndForLoansModifiedAsTroubledDebtRestructurings
Namespace Prefix:	mbwm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

X

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

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

X

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524

+ Details

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