| **Disclosures About Fair Value of Assets and Liabilities - Fair Value Measurements of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                                                                                                                          | Dec. 31, 2014                                                                                                                                                                                                                                                                                                                                          |
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| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                                           | $ 538,055us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                            | $ 549,543us-gaap_AvailableForSaleSecurities                                                                                                                                                                                                                                                                                                            |
| Significant Other Observable Inputs (Level 2)                                                                                                                                                                      |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                                           | 531,857us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                                                         | 542,897us-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                                                                                                                                                                           | 6,198us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                                                           | 6,646us-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                                                                                                                                                                                              | 8,569us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                   | 6,537us-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. Government-sponsored agency securities                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                                           | 107us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                             | 109us-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                                                                                                                                                                           | 255,421us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                           | 221,982us-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                                                                                                                                                                           | 274,627us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember           | 319,104us-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                                                                                                                                                                                                  | 4,764us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                | 3,730us-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                                                                                                                                                                                                  | 87us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                                       | 137us-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                                                                                                                                                                           | 6,163us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                             | 6,611us-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                                                         | 31us-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                                                                                                                                                                                              | 8,569us-gaap_DerivativeFairValueOfDerivativeLiability/ us-gaap_FairValueByLiabilityClassAxis= frme_InterestRateSwapLiabilitiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                         | 6,537us-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. Government-sponsored agency securities                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| **Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]**                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                   |
| Investment securities available for sale                                                                                                                                                                           | 107us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                   | 109us-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                                                                                                                                                                           | 261,584us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                 | 228,593us-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                                                                                                                                                                           | 274,627us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByAssetClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember | 319,104us-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                                               | 31us-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                                                                                                                                                                                                  | 4,764us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= frme_InterestRateSwapAssetsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                      | 3,730us-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                                                                                                                                                                                                  | $ 87us-gaap_DerivativeFairValueOfDerivativeAsset/ us-gaap_FairValueByAssetClassAxis= us-gaap_InterestRateCapMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsRecurringMember                                           | $ 137us-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
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| 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
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| 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
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