Mortgage Servicing Rights and Mortgage Servicing Activity (Changes in the Carrying Amount of Mortgage Servicing Rights) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 12, 2013
Mortage Servicing Rights Amortized Value [Roll Forward]
Mortgage Servicing Rights Beginning of Period	$ 22,760fmer_MortgageServicingRights	$ 21,316fmer_MortgageServicingRights	$ 21,179fmer_MortgageServicingRights
Mortgage Servicing Rights Additions	3,049fmer_MortgageServicingRightsAdditions	4,952fmer_MortgageServicingRightsAdditions	5,876fmer_MortgageServicingRightsAdditions
Amortization of Mortgage Servicing Rights (MSRs)	(3,798)	(4,573)	(5,739)
Mortgage Servicing Rights, End of Period	22,011fmer_MortgageServicingRights	22,760fmer_MortgageServicingRights	21,316fmer_MortgageServicingRights
Mortage Servicing Rights Valuation Rollforward [Roll Forward]
Mortgage Servicing rights Valuation Allowance Beginning of Period	(282)	(2,564)	(3,539)
Mortgage Servicing Rights (MSR) Impairment (Recovery)	(673)	2,282	975
Mortgage Servicing rights Valuation Allowance, End of Period	955	(282)	(2,564)
MSRs, net carrying balance	21,056	22,478	18,752
Fair value at end of period	21,228fmer_FairValueOfMortgageServicingRights	23,041fmer_FairValueOfMortgageServicingRights
Citizens Republic Bancorp
Mortage Servicing Rights Amortized Value [Roll Forward]
Addition of Citizens' MSR's on Acquisition Date				681,100
Mortgage Servicing Rights [Member] \| Citizens Republic Bancorp
Mortage Servicing Rights Amortized Value [Roll Forward]
Addition of Citizens' MSR's on Acquisition Date	$ 0		$ 0	$ 1,065

Fair Value of Mortgage Servicing Rights

No definition available.

Name:	fmer_FairValueOfMortgageServicingRights
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Mortgage Servicing Rights.

+ References

No definition available.

+ Details

Name:	fmer_MortgageServicingRights
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Mortgage Servicing Rights Additions

+ References

No definition available.

+ Details

Name:	fmer_MortgageServicingRightsAdditions
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Mortgage Servicing Rights Net Carrying balance

+ References

No definition available.

+ Details

Name:	fmer_MortgageServicingRightsNetCarryingBalance
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Mortgage Servicing rights Valuation Allowance

+ References

No definition available.

+ Details

Name:	fmer_MortgageServicingRightsValuationAllowance
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	fmer_ServicingAssetValuationRollforwardRollForward
Namespace Prefix:	fmer_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The periodic amortization (in proportion to and over the period of estimated net servicing income or loss) of capitalized servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 4

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122739-111746

+ Details

Name:	us-gaap_AmortizationOfMortgageServicingRightsMSRs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The adjustment to the carrying value of the rights retained or purchased to service mortgages. These adjustments are made when the estimate of the fair value is changed. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 4

-Subparagraph (a)(6),(7)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122739-111746

+ Details

Name:	us-gaap_MortgageServicingRightsMSRImpairmentRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmer_CitizensRepublicBancorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=fmer_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: