| **Pension and Other Post Retirement Benefit Plans - Schedule of Plan Assets by Level (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                                              | Dec. 31, 2013                                                                                                                                                                                                                                              | Dec. 31, 2012                                           |
|--------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------|
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | $ 77,139us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                                                                    | $ 69,871us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                                                                                                                                    | $ 62,865us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
| Quoted Prices in Active Markets for Identical Assets (Level 1)                                                                                   |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 57,413us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                 | 51,176us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                 |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents                                                       |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 2,032us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CashAndCashEquivalentsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                       | 1,864us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CashAndCashEquivalentsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                       |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds and Notes                                                       |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 9,384us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                      | 6,076us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                      |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 1,376us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CommonStockMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                  | 1,375us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CommonStockMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                  |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Taxable Bond                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 4,987us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_TaxableBondMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                     | 3,901us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_TaxableBondMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                     |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Large Cap Equity                                                                |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 21,185us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_LargeCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                 | 20,617us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_LargeCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                 |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Mid Cap Equity                                                                  |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 9,434us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_MidCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                    | 8,721us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_MidCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                    |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Small Cap Equity                                                                |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 3,872us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SmallCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                  | 3,584us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SmallCapEquityMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                  |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | International Equity                                                            |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 3,474us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_InternationalMutualFundsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                        | 3,727us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_InternationalMutualFundsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                        |                                                    |
| Quoted Prices in Active Markets for Identical Assets (Level 1) | Specialty Alternative Equity                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 1,669us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SpecialtyAlternativeFundsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                       | 1,311us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SpecialtyAlternativeFundsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                       |                                                    |
| Significant Other Observable Inputs (Level 2)                                                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 19,726us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                 | 18,695us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                 |                                                    |
| Significant Other Observable Inputs (Level 2) | Government Agency and Municipal Bonds and Notes                                                  |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 8,252us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member           | 8,263us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member           |                                                    |
| Significant Other Observable Inputs (Level 2) | Certificates of Deposit                                                                          |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 1,001us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CertificatesOfDepositMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                        | 607us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CertificatesOfDepositMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                          |                                                    |
| Significant Other Observable Inputs (Level 2) | Common Bond Fund                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 4,615us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonBondFundMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                  | 5,318us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonBondFundMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                  |                                                    |
| Significant Other Observable Inputs (Level 2) | Common Equity Fund                                                                               |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 5,858us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonEquityFundMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                | 4,507us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonEquityFundMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                |                                                    |
| Significant Unobservable Inputs (Level 3)                                                                                                        |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                      | 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                      |                                                    |
| Fair Value                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 77,139us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                                                                                                       | 69,871us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                                                                                                       |                                                    |
| Fair Value | Cash and cash equivalents                                                                                                           |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 2,032us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CashAndCashEquivalentsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember             | 1,864us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CashAndCashEquivalentsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember             |                                                    |
| Fair Value | Corporate Bonds and Notes                                                                                                           |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 9,384us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember            | 6,076us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CorporateDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember            |                                                    |
| Fair Value | Government Agency and Municipal Bonds and Notes                                                                                     |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 8,252us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember | 8,263us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember |                                                    |
| Fair Value | Certificates of Deposit                                                                                                             |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 1,001us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CertificatesOfDepositMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember              | 607us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CertificatesOfDepositMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                |                                                    |
| Fair Value | Common Stock                                                                                                                        |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 1,376us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CommonStockMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        | 1,375us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= us-gaap_CommonStockMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        |                                                    |
| Fair Value | Common Bond Fund                                                                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 4,615us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonBondFundMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        | 5,318us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonBondFundMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        |                                                    |
| Fair Value | Common Equity Fund                                                                                                                  |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 5,858us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonEquityFundMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                      | 4,507us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_CommonEquityFundMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                      |                                                    |
| Fair Value | Taxable Bond                                                                                                                        |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 4,987us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_TaxableBondMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                           | 3,901us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_TaxableBondMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                           |                                                    |
| Fair Value | Large Cap Equity                                                                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 21,185us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_LargeCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                       | 20,617us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_LargeCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                       |                                                    |
| Fair Value | Mid Cap Equity                                                                                                                      |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 9,434us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_MidCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                          | 8,721us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_MidCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                          |                                                    |
| Fair Value | Small Cap Equity                                                                                                                    |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 3,872us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SmallCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        | 3,584us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SmallCapEquityMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember                        |                                                    |
| Fair Value | International Equity                                                                                                                |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | 3,474us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_InternationalMutualFundsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember              | 3,727us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_InternationalMutualFundsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember              |                                                    |
| Fair Value | Specialty Alternative Equity                                                                                                        |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                                                 |                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                       |                                                    |
| Fair Value of Plan Assets                                                                                                                        | $ 1,669us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SpecialtyAlternativeFundsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember           | $ 1,311us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis= frme_SpecialtyAlternativeFundsMember/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EstimateOfFairValueFairValueDisclosureMember           |                                                    |

| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlanDisclosureLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930
+ Details**Name:**us-gaap_DefinedBenefitPlanFairValueOfPlanAssets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| 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:** |