Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
OPERATIONS:
Net interest income	$ 185,623us-gaap_InterestIncomeExpenseNet	$ 193,900us-gaap_InterestIncomeExpenseNet
Provision for Loan, Lease, and Other Losses	8,248	14,536
Noninterest Income	65,847us-gaap_NoninterestIncome	67,270us-gaap_NoninterestIncome
Other expenses	160,652us-gaap_NoninterestExpense	169,331us-gaap_NoninterestExpense
Net income	57,139us-gaap_NetIncomeLoss	53,455us-gaap_NetIncomeLoss
AVERAGES :
Assets	24,905,094	24,144,570
Segment Reporting Information Average Loans	15,427,181	14,412,481
Segment Reporting Information Average Earnings Assets	22,100,417	20,903,863
Segment Reporting Information Average Deposits	19,788,925	19,636,505
Segment Reporting Information Average Economic Capital	2,866,362	2,733,226
Total commercial
OPERATIONS:
Net interest income	99,844	105,063
Provision for Loan, Lease, and Other Losses	(526)	5,706
Noninterest Income	22,490	21,306
Other expenses	61,653	63,966
Net income	39,785	36,884
AVERAGES :
Assets	9,454,218	9,072,390
Segment Reporting Information Average Loans	9,506,529	9,061,371
Segment Reporting Information Average Earnings Assets	9,794,483	9,308,031
Segment Reporting Information Average Deposits	6,886,945	6,645,640
Segment Reporting Information Average Economic Capital	848,890	654,922
Retail
OPERATIONS:
Net interest income	91,095	93,250
Provision for Loan, Lease, and Other Losses	7,533	8,339
Noninterest Income	21,737	27,035
Other expenses	88,271	96,701
Net income	11,068	9,909
AVERAGES :
Assets	5,845,417	5,449,856
Segment Reporting Information Average Loans	5,570,590	5,084,555
Segment Reporting Information Average Earnings Assets	5,582,849	5,101,740
Segment Reporting Information Average Deposits	11,124,158	11,753,015
Segment Reporting Information Average Economic Capital	479,188	320,090
Wealth
OPERATIONS:
Net interest income	5,353	4,678
Provision for Loan, Lease, and Other Losses	(170)	(45)
Noninterest Income	13,976	13,464
Other expenses	13,719	13,518
Net income	3,756	3,035
AVERAGES :
Assets	302,186	241,618
Segment Reporting Information Average Loans	292,016	230,429
Segment Reporting Information Average Earnings Assets	292,016	230,429
Segment Reporting Information Average Deposits	1,240,960	1,009,811
Segment Reporting Information Average Economic Capital	55,187	57,784
Nonreportable segments and reconciling items
OPERATIONS:
Net interest income	(10,669)	(9,091)
Provision for Loan, Lease, and Other Losses	1,411	536
Noninterest Income	7,644	5,465
Other expenses	(2,991)	(4,854)
Net income	2,530	3,627
AVERAGES :
Assets	9,303,273	9,380,706
Segment Reporting Information Average Loans	58,046	36,126
Segment Reporting Information Average Earnings Assets	6,431,069	6,263,663
Segment Reporting Information Average Deposits	536,862	228,039
Segment Reporting Information Average Economic Capital	$ 1,483,097	$ 1,700,430

SegmentReportingInformationAverageAssets1

No definition available.

Name:	fmer_Segmentreportinginformationaverageassets1
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Segment Reporting Information Average Deposits

+ References

No definition available.

+ Details

Name:	fmer_SegmentReportingInformationAverageDeposits
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Segment Reporting Information Average Earnings Assets

+ References

No definition available.

+ Details

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

- Definition

Segment Reporting Information Average Economic Capital

+ References

No definition available.

+ Details

Name:	fmer_SegmentReportingInformationAverageEconomicCapital
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Segment Reporting Information Average Loans

+ References

No definition available.

+ Details

Name:	fmer_SegmentReportingInformationAverageLoans
Namespace Prefix:	fmer_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

The net amount of operating interest income (expense).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.10)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 10

-Article 9

+ Details

Name:	us-gaap_InterestIncomeExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ 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 260

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.19)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.18)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.22)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 19

-Article 5

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 20

-Article 9

+ Details

Name:	us-gaap_NetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total aggregate amount of all noninterest expense.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 14

-Article 9

+ Details

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

- Definition

The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Article 9

+ Details

Name:	us-gaap_NoninterestIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ 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 (a)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fmer_NonreportableSegmentsAndReconcilingItemsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fmer_RetailMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fmer_WealthMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: