Note 9 - Stock-based Compensation (Details) - Option Information (USD $)	6 Months Ended	12 Months Ended	1 Months Ended	6 Months Ended
Jun. 30, 2014 Mercantile Plans [Member]	Dec. 31, 2013 Mercantile Plans [Member]	Dec. 31, 2012 Mercantile Plans [Member]	Jun. 30, 2014 Firstbank Plans [Member]	Jun. 30, 2014 Firstbank Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum exercise price	$ 6.21	$ 6.21	$ 6.21	$ 7.80
Maximum exercise price	$ 40.28	$ 40.28	$ 40.28	$ 24.46
Average remaining option term (years)	328 days	1 year 146 days	1 year 328 days		3 years 109 days
Aggregate intrinsic value of stock options exercised	$ 11,000	$ 408,000	$ 307,000	$ 40,000
Cash received from stock option exercises	50,000	289,000	227,000	24,000
Tax benefit realized from stock option exercises	$ 0	$ 0	$ 0	$ 0
Weighted average per share fair value of stock options granted

Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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

-Publisher FASB

-Name Accounting Standards Codification

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The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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