Note 9 - Stock-based Compensation (Details) - Options by Exercise Price Range (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended	12 Months Ended	1 Months Ended	6 Months Ended
Jun. 30, 2014 Exercise Price Range 1 [Member] Mercantile Plans [Member]	Jun. 30, 2014 Exercise Price Range 1 [Member] Firstbank Plans [Member]	Jun. 30, 2014 Exercise Price Range 2 [Member] Mercantile Plans [Member]	Jun. 30, 2014 Exercise Price Range 2 [Member] Firstbank Plans [Member]	Jun. 30, 2014 Exercise Price Range 3 [Member] Mercantile Plans [Member]	Jun. 30, 2014 Exercise Price Range 3 [Member] Firstbank Plans [Member]	Jun. 30, 2014 Exercise Price Range 4 [Member] Firstbank Plans [Member]	Jun. 30, 2014 Exercise Price Range 5 [Member] Firstbank Plans [Member]	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, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower limit	$ 6.21	$ 4.00	$ 32.01	$ 8.01	$ 40.01	$ 12.01	$ 20.01	$ 24.01	$ 6.21	$ 6.21	$ 6.21	$ 7.80
Range of exercise prices, upper limit	$ 8.00	$ 8.00	$ 36.00	$ 12.00	$ 44.00	$ 16.00	$ 24.00	$ 28.00	$ 40.28	$ 40.28	$ 40.28	$ 24.46
Options, outstanding (in Shares)	2,700	78,855	51,300	31,830	2,890	46,000	85,962	36,456	56,890			279,103	279,103
Options, weighted average remaining contractual life	1 year 146 days	5 years 146 days	328 days	5 years 146 days	109 days	3 years 146 days	1 year 328 days	146 days	328 days	1 year 146 days	1 year 328 days		3 years 109 days
Options, outstanding weighted average exercise price	$ 6.21	$ 6.54	$ 34.87	$ 8.60	$ 40.28	$ 16.00	$ 22.43	$ 24.46	$ 33.79			$ 15.57	$ 15.57
Options, exercisable (in Shares)	2,700	62,215	51,300	24,740	2,890	46,000	85,962	36,456	56,890			255,373	255,373
Options, exercisable weighted average exercise price	$ 6.21	$ 6.90	$ 34.87	$ 8.60	$ 40.28	$ 16.00	$ 22.43	$ 24.46	$ 33.79			$ 16.44	$ 16.44

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration