| **Fair Values of Financial Instruments - Fair Value Measurements of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                                                                                                                                          | Dec. 31, 2013                                                                                                                                                                                                                                                                                                                                          |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | $ 549,543us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                            | $ 536,201us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                            |
| Significant Other Observable Inputs (Level 2)                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 542,897us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                                                         | 526,224us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                                                         |
| Significant Unobservable Inputs (Level 3)                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 6,646us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                                                           | 9,977us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                                                           |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest rate swap liability                                                                              |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Liability derivatives                                                                                                                                                                           | 6,537us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                   | 3,953us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                   |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury                                                                                             |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                   | 15,973us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USTreasurySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                        |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government-sponsored agency securities                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 109us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                             | 3,545us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                           |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | State and municipal                                                                                       |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 221,982us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                           | 223,752us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                           |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government-sponsored mortgage-backed securities                                                      |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 319,104us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember           | 281,252us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember           |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Equity securities                                                                                         |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 1,702us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                             | 1,702us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                             |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest rate swap asset                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Asset derivatives                                                                                                                                                                               | 3,730us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                | 3,619us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                |
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest rate cap                                                                                         |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Asset derivatives                                                                                                                                                                               | 137us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                      | 390us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                      |
| Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | State and municipal                                                                                           |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 6,611us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                             | 7,235us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                             |
| Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate obligations                                                                                         |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 31us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                         | 2,738us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                      |
| Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Equity securities                                                                                             |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 4us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                 | 4us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                                 |
| Fair Value | Fair Value, Measurements, Recurring | Interest rate swap liability                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Liability derivatives                                                                                                                                                                           | 6,537us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                         | 3,953us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                         |
| Fair Value | Fair Value, Measurements, Recurring | U.S. Treasury                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                   | 15,973us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USTreasurySecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                              |
| Fair Value | Fair Value, Measurements, Recurring | U.S. Government-sponsored agency securities                                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 109us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                   | 3,545us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                 |
| Fair Value | Fair Value, Measurements, Recurring | State and municipal                                                                                                                          |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 228,593us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                 | 230,987us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                 |
| Fair Value | Fair Value, Measurements, Recurring | U.S. Government-sponsored mortgage-backed securities                                                                                         |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 319,104us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember | 281,252us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember |
| Fair Value | Fair Value, Measurements, Recurring | Corporate obligations                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 31us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                               | 2,738us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                            |
| Fair Value | Fair Value, Measurements, Recurring | Equity securities                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                        | 1,706us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                   | 1,706us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                   |
| Fair Value | Fair Value, Measurements, Recurring | Interest rate swap asset                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Asset derivatives                                                                                                                                                                               | 3,730us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                      | 3,619us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                      |
| Fair Value | Fair Value, Measurements, Recurring | Interest rate cap                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                               |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Asset derivatives                                                                                                                                                                               | $ 137us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                          | $ 390us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313
+ Details**Name:**us-gaap_DerivativeFairValueOfDerivativeAsset**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313
+ Details**Name:**us-gaap_DerivativeFairValueOfDerivativeLiability**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |