Interest rate of Non-Cumulative Perpetual Preferred Stock, Series A

No definition available.

Name:	fmer_InterestrateofNonCumulativePerpetualPreferredStockSeriesA
Namespace Prefix:	fmer_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Non-cumulative Perpetual Preferred Stock, Series A, Shares Authorized

+ References

No definition available.

+ Details

Name:	fmer_NoncumulativePerpetualPreferredStockSeriesASharesAuthorized
Namespace Prefix:	fmer_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Non-cumulative Perpetual Preferred Stock, Shares Outstanding

+ References

No definition available.

+ Details

Name:	fmer_NoncumulativePerpetualPreferredStockSharesOutstanding
Namespace Prefix:	fmer_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(i)(2))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph i

-Subparagraph 2

-Article 4

+ Details

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

- Definition

The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.28)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29

-Article 5

+ Details

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

- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

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

+ Details

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

- Details

Name:	dei_LegalEntityAxis=fmer_CitizensRepublicBancorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=fmer_FirstMeritMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Shareholders' Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 04, 2013	Apr. 12, 2013
Business Acquisition [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights	0
Preferred Stock, Shares Authorized	7,000,000us-gaap_PreferredStockSharesAuthorized
Non-cumulative Perpetual Preferred Stock, Series A, Shares Authorized	115,000
Non-cumulative Perpetual Preferred Stock, Shares Outstanding	100,000
Outstanding, Intrinsic Value	$ 0.8	$ 1.4	$ 3.4
Citizens Republic Bancorp
Business Acquisition [Line Items]
Interest rate of Non-Cumulative Perpetual Preferred Stock, Series A				5.875%
FirstMerit \| Citizens Republic Bancorp
Business Acquisition [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights	2,549,701.6				2,408,203.0
Citizens Republic Bancorp \| Citizens Republic Bancorp
Business Acquisition [Line Items]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					1,757,812.5
Class of Warrant or Right, Exercise Price of Warrants or Rights	$ 17.65