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Note 5 - Deposits (Details Textual) - USD ($) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2025 | Dec. 31, 2024
Deposits | $ 4,811,813 | $ 4,698,366
Increase (Decrease) in Deposits | $ 113,000 |
Aggregate Increase (Decrease) in Percentage of Deposits | 2.40% |

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- Definition The percentage of increase or (decrease) during the period in deposit liabilities. + References No definition available. + Details
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Namespace Prefix: | mbwm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
Name: | us-gaap_Deposits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The net cash inflow or outflow for the increase (decrease) in the beginning and end of period deposits balances. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 230 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479024/942-230-45-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-8 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration