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Note 5 - Deposits (Details Textual) - USD ($) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Deposits, Total | $ 2,529,811,000 | $ 2,522,365,000
Increase (Decrease) in Deposits, Total | $ 7,400,000 |
Aggregate Increase (Decrease) in Percentage of Deposits | 0.30% |
Time Deposits, at or Above FDIC Insurance Limit | $ 215,000,000 | $ 266,000,000

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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: | 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://fasb.org/us-gaap/role/ref/legacyRef -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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- 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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3095-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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Balance Type: | debit
Period Type: | duration

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- Definition Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=50488274&loc=d3e64164-112818 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant