| **Pension and Other Post Retirement Benefit Plans - Other Comprehensive Income (Details) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                                                                             |
|--------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| **Defined Benefit Plan Disclosure [Line Items]**                                                                                           |                                                                                                                                                                                                                                                        |   |   |
| Net periodic pension (benefit) cost                                                                                                        | $ 600us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost                                                                                                                                                                                                       | $ (692)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | $ (1,167)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost |
| Pension Plans                                                                                                                              |                                                                                                                                                                                                                                                        |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                           |                                                                                                                                                                                                                                                        |   |   |
| Net periodic pension (benefit) cost                                                                                                        | (600)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                                              | 692us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 1,167us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Net gain (loss)                                                                                                                            | (12,203)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember     | 11,942us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | (1,242)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Amortization of loss                                                                                                                       | 478us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                 | 2,131us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 2,207us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Amortization of prior service cost                                                                                                         | 81us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 25us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 25us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Total recognized in other comprehensive income (loss)                                                                                      | (11,644)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember             | 14,098us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | 990us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |
| Total recognized in net periodic pension cost and other comprehensive income (loss)                                                        | $ (11,044)frme_DefinedBenefitPlanRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTaxes/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                      | $ 13,406frme_DefinedBenefitPlanRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTaxes/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember | $ (177)frme_DefinedBenefitPlanRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTaxes/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Recognized in Net Periodic Benefit Cost and Other Comprehensive Income (Loss), Before Taxes
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax and reclassification adjustments of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement benefit plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e689-108580Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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