| **Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                   |                                                                                                                                                                                                                  |
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| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | $ 136,861us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance                                                                                                                         |                                                                                                                                                                                                             | $ 158,563us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance |
| Impaired loans with related allowance - Unpaid Principal Balance                                                                | 12,896us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance                                                                                                                              |                                                                                                                                                                                                             | 16,703us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance |
| Unpaid Principal Balance                                                                                                        | 149,757us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance                                                                                                                                                 |                                                                                                                                                                                                             | 175,266us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 93,646us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment                                                                                                                                |                                                                                                                                                                                                             | 102,793us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment |
| Impaired loans with related allowance - Recorded Investment                                                                     | 10,904us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment                                                                                                                                  |                                                                                                                                                                                                             | 13,430us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment |
| Recorded Investment                                                                                                             | 104,550us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                                                     |                                                                                                                                                                                                             | 116,223us-gaap_ImpairedFinancingReceivableRecordedInvestment |
| Impaired financing receivable, allowance                                                                                        | 4,556us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                                                         | 1,794us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                                                         | 2,769us-gaap_ImpairedFinancingReceivableRelatedAllowance |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 103,092us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment                                                                                                                        | 112,759us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment                                                                                                                        |   |
| Impaired loans with related allowance - Average Recorded Investment                                                             | 13,244us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment                                                                                                                           | 6,150us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment                                                                                                                            |   |
| Average Recorded Investment                                                                                                     | 116,336us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment                                                                                                                                              | 118,909us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment                                                                                                                                              |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 1,172us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                        | 300us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                          |   |
| Impaired loans with related allowance - Interest Income Recognized                                                              | 10us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                             | 4us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                              |   |
| Interest Income Recognized                                                                                                      | 1,182us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod                                                                                                                                              | 304us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod                                                                                                                                                |   |
| Commercial and industrial loans                                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 27,308us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    |                                                                                                                                                                                                             | 35,514us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Impaired loans with related allowance - Unpaid Principal Balance                                                                | 4,434us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember       |                                                                                                                                                                                                             | 1,766us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 13,460us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember        |                                                                                                                                                                                                             | 18,029us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Impaired loans with related allowance - Recorded Investment                                                                     | 4,376us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember           |                                                                                                                                                                                                             | 1,684us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Impaired financing receivable, allowance                                                                                        | 2,461us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                 |                                                                                                                                                                                                             | 1,055us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 16,150us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 16,377us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |   |
| Impaired loans with related allowance - Average Recorded Investment                                                             | 4,379us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    | 1,752us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 139us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember  | 21us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember   |   |
| Impaired loans with related allowance - Interest Income Recognized                                                              | 10us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember     | 3us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember      |   |
| Agricultural production financing and other loans to farmers                                                                    |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 307us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember              |                                                                                                                                                                                                             | 26us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |
| Impaired loans with related allowance - Unpaid Principal Balance                                                                | 3,415us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember              |                                                                                                                                                                                                             | 6,777us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 302us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                  |                                                                                                                                                                                                             | 22us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |
| Impaired loans with related allowance - Recorded Investment                                                                     | 1,676us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                  |                                                                                                                                                                                                             | 5,777us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |
| Impaired financing receivable, allowance                                                                                        | 239us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                          |                                                                                                                                                                                                             | 400us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 303us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember           | 29us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember            |   |
| Impaired loans with related allowance - Average Recorded Investment                                                             | 3,991us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember           |                                                                                                                                                                                                             |   |
| Real estate loans - Construction                                                                                                |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 10,811us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember            |                                                                                                                                                                                                             | 12,956us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 7,856us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                 |                                                                                                                                                                                                             | 9,318us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 8,542us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 10,440us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember         |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 104us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 28us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember           |   |
| Real estate loans - Commercial and farmland                                                                                     |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 88,069us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      |                                                                                                                                                                                                             | 95,856us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Impaired loans with related allowance - Unpaid Principal Balance                                                                | 3,171us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember         |                                                                                                                                                                                                             | 7,159us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 65,281us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember          |                                                                                                                                                                                                             | 68,187us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Impaired loans with related allowance - Recorded Investment                                                                     | 3,093us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember             |                                                                                                                                                                                                             | 4,971us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Impaired financing receivable, allowance                                                                                        | 1,302us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                   |                                                                                                                                                                                                             | 1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 69,036us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember   | 81,617us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember   |   |
| Impaired loans with related allowance - Average Recorded Investment                                                             | 3,113us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      | 4,398us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 878us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember    | 243us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember    |   |
| Impaired loans with related allowance - Interest Income Recognized                                                              |                                                                                                                                                                                                        | 1us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember        |   |
| Real estate loans - Residential                                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 9,792us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         |                                                                                                                                                                                                             | 10,591us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired loans with related allowance - Unpaid Principal Balance                                                                | 1,876us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember           |                                                                                                                                                                                                             | 1,001us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 6,273us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember             |                                                                                                                                                                                                             | 6,839us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired loans with related allowance - Recorded Investment                                                                     | 1,759us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember               |                                                                                                                                                                                                             | 998us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired financing receivable, allowance                                                                                        | 554us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                       |                                                                                                                                                                                                             | 194us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 8,528us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember      | 4,146us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember      |   |
| Impaired loans with related allowance - Average Recorded Investment                                                             | 1,761us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember        |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 48us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       | 8us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember        |   |
| Impaired loans with related allowance - Interest Income Recognized                                                              |                                                                                                                                                                                                        |                                                                                                                                                                                                             |   |
| Real estate loans - Home equity                                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 546us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                      |                                                                                                                                                                                                             | 3,590us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  | 474us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                          |                                                                                                                                                                                                             | 398us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Impaired loans with no related allowance - Average Recorded Investment                                                          | 533us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                   | 144us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                   |   |
| Impaired loans with no related allowance - Interest Income Recognized                                                           | 3us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                   |                                                                                                                                                                                                        |   |
| Other loans                                                                                                                     |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                 |                                                                                                                                                                                                             |                                                                                                                                                                                                             |   |
| Impaired loans with no related allowance - Unpaid Principal Balance                                                             | 28us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                          |                                                                                                                                                                                                             | 30us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember |
| Impaired loans with no related allowance - Recorded Investment                                                                  |                                                                                                                                                                                                        |                                                                                                                                                                                                             |    |
| Impaired loans with no related allowance - Average Recorded Investment                                                          |                                                                                                                                                                                                             | $ 6us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                      |   |

| X                                                                                                                                                                        |
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| - Details**Name:**us-gaap_FinancingReceivableImpairedLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.
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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
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Amount of allowance for credit losses related to recorded investment.
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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
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Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.
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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
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Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.
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Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.
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| - Definition
Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.
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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.
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| - Definition
Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
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