Note 2 - Business Combination (Details) - Purchase Price (Firstbank Corporation [Member], USD $)	0 Months Ended	1 Months Ended
Jun. 01, 2014	Jun. 30, 2014
Firstbank Corporation [Member]
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Mercantile common shares issued for Firstbank common shares (in Shares)	8,087,272	8,087,272
Price per share, based on Mercantile closing price on May 30, 2014 (in Dollars per share)		$ 21.43
Value of common stock issued		$ 173,310,000
Replacement stock options		1,664,000
Total purchase price		$ 174,974,000

Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)(4)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

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- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

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- Definition

Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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No definition available.

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- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

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Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

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