Note 6 - Deposits (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Disclosure Text Block [Abstract]
Deposits	$ 2,279,524,000us-gaap_Deposits	$ 2,276,915,000us-gaap_Deposits
Increase (Decrease) in Deposits	$ 2,600,000us-gaap_IncreaseDecreaseInDeposits
Percentage of Increase in Aggregate Deposits	0.10%

Item represents the percentage of increase in aggregate deposits.

No definition available.

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

- 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

- Details

Name:	us-gaap_DisclosureTextBlockAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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