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Note 5 - Deposits (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014
Disclosure Text Block [Abstract] |  |
Deposits | $ 2,254,129,000 | $ 2,276,915,000
Increase (Decrease) in Deposits | $ (22,800,000) |
Percentage of Increase in Aggregate Deposits | (1.00%) |

X
- Definition Item represents the percentage of increase in aggregate deposits. + References No definition available. + Details
Name: | mbwm_PercentageOfIncreaseInAggregateDeposits
Namespace Prefix: | mbwm_
Data Type: | num: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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.12) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_Deposits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DisclosureTextBlockAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 230 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6477933&loc=d3e60009-112784 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3095-108585 + Details
Name: | us-gaap_IncreaseDecreaseInDeposits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration