Changes and Reclassifications Out of Accumulated Other Comprehensive Income (Tax Effect of Changes and Reclassifications Out of Accumulated Other Comprehensive Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	$ (75,792)	$ (85,390)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, before Tax	354	56
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax	124	20
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Net of Tax	(230)	(36)
Accumulated other comprehensive loss	(49,267)	(55,504)	(71,892)
accumulated other comprehensive income, tax	(26,525)	(29,886)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	(100,520)	(57,812)	(102,068)	(57,812)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, Tax	(176)	(173)
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	24,728	(27,578)	(8,531)	(45,072)
Accumulated other comprehensive loss	16,072	(17,925)	(5,546)	(29,297)
accumulated other comprehensive income, tax	$ 8,656	$ (9,653)	$ (2,985)	$ (15,775)

Changes and Reclassifications Out of Accumulated Other Comprehensive Income (Tax Effect of Changes and Reclassifications Out of Accumulated Other Comprehensive Income) (Details) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Accumulated Other Comprehensive Income (Loss), before Tax

$ (75,792)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1

$ (85,390)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1

Net losses/(gains) realized on sale of securities reclassified to noninterest income, before Tax

354us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax

56us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax

Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax

124us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax

20us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax

Net losses/(gains) realized on sale of securities reclassified to noninterest income, Net of Tax

(230)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax

(36)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax

Accumulated other comprehensive loss

(49,267)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax

(55,504)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax

(71,892)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax

accumulated other comprehensive income, tax

(26,525)fmer_Accumulatedothercomprehensiveincometax

(29,886)fmer_Accumulatedothercomprehensiveincometax

Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax

(100,520)us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax

(57,812)us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax

(102,068)us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax

(57,812)us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax

Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, Tax

(176)us-gaap_OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesTax

(173)us-gaap_OtherComprehensiveIncomeReclassificationAdjustmentForHeldToMaturityTransferredToAvailableForSaleSecuritiesTax

Accumulated Net Unrealized Investment Gain (Loss)

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Accumulated Other Comprehensive Income (Loss), before Tax

24,728us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
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(27,578)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
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(8,531)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
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(45,072)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
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Accumulated other comprehensive loss

16,072us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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(17,925)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
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(5,546)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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(29,297)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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accumulated other comprehensive income, tax

$ 8,656fmer_Accumulatedothercomprehensiveincometax
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$ (9,653)fmer_Accumulatedothercomprehensiveincometax
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$ (2,985)fmer_Accumulatedothercomprehensiveincometax
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= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember

$ (15,775)fmer_Accumulatedothercomprehensiveincometax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember

- Definition

accumulated other comprehensive income, tax

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Amount before tax of increase (decrease) in accumulated in equity from transactions and other events and circumstances from non-owner sources. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The pretax total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.

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Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) related to transfers of securities into the available-for-sale category and out of the held-to-maturity category.

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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