| **Loans and Allowance - Composition of Impaired Loans by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                                  |
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| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | $ 158,563us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance                                                                                                                         | $ 189,779us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance | $ 109,594us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance |
| Recorded Investment                                                                                                            | 102,793us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment                                                                                                                               | 114,253us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment | 67,333us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment |
| Average Recorded Investment                                                                                                    | 106,807us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment                                                                                                                        | 119,561us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment | 76,517us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment |
| Interest Income Recognized                                                                                                     | 4,297us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                        | 2,048us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod | 1,498us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod |
| Unpaid Principal Balance                                                                                                       | 16,703us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance                                                                                                                              | 6,121us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance | 12,918us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance |
| Recorded Investment                                                                                                            | 13,430us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment                                                                                                                                  | 5,502us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment | 11,846us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment |
| Related Allowance                                                                                                              | 2,769us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                                                         | 1,583us-gaap_ImpairedFinancingReceivableRelatedAllowance | 4,243us-gaap_ImpairedFinancingReceivableRelatedAllowance |
| Average Recorded Investment                                                                                                    | 15,764us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment                                                                                                                           | 5,607us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment | 12,097us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment |
| Interest Income Recognized                                                                                                     | 65us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod                                                                                                                             | 18us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod | 203us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod |
| Impaired Financing Receivable, Unpaid Principal Balance                                                                        | 175,266us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance                                                                                                                                                 | 195,900us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance | 122,512us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance |
| Impaired Financing Receivable, Recorded Investment                                                                             | 116,223us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                                                     | 119,755us-gaap_ImpairedFinancingReceivableRecordedInvestment | 79,179us-gaap_ImpairedFinancingReceivableRecordedInvestment |
| Average Recorded Investment                                                                                                    | 122,571us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment                                                                                                                                              | 125,168us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | 88,614us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment |
| Interest Income Recognized                                                                                                     | 4,362us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod                                                                                                                                              | 2,066us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | 1,701us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod |
| Commercial and industrial loans                                                                                                |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 35,514us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    | 35,066us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 28,532us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Recorded Investment                                                                                                            | 18,029us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember        | 16,371us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 11,730us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Average Recorded Investment                                                                                                    | 18,711us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 19,209us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 15,089us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Interest Income Recognized                                                                                                     | 362us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember  | 192us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 124us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Unpaid Principal Balance                                                                                                       | 1,766us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember       | 1,390us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 4,415us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Recorded Investment                                                                                                            | 1,684us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember           | 1,216us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 4,155us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Related Allowance                                                                                                              | 1,055us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                 | 683us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 1,628us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Average Recorded Investment                                                                                                    | 1,721us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    | 1,240us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 4,225us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Interest Income Recognized                                                                                                     | 40us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember     | 9us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 33us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Agriculture production financing and other loans to farmers                                                                    |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 26us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember               | 32us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |   |
| Recorded Investment                                                                                                            | 22us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                   | 30us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |   |
| Average Recorded Investment                                                                                                    | 26us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember            | 32us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember |   |
| Unpaid Principal Balance                                                                                                       | 6,777us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember              |   |   |
| Recorded Investment                                                                                                            | 5,777us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                  |   |   |
| Related Allowance                                                                                                              | 400us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                          |   |   |
| Average Recorded Investment                                                                                                    | 8,044us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember           |   |   |
| Interest Income Recognized                                                                                                     | 1us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember             |   |   |
| Construction                                                                                                                   |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 12,956us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember            | 16,109us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 9,787us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Recorded Investment                                                                                                            | 9,318us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                 | 10,625us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 5,164us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Average Recorded Investment                                                                                                    | 9,837us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 11,621us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 6,471us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Interest Income Recognized                                                                                                     | 427us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 117us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 66us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Unpaid Principal Balance                                                                                                       |                                                                                                                                                                                                             | 4,657us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 1,202us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Recorded Investment                                                                                                            |                                                                                                                                                                                                             | 4,215us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 1,058us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Related Allowance                                                                                                              |                                                                                                                                                                                                             | 894us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 105us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Average Recorded Investment                                                                                                    |                                                                                                                                                                                                             | 4,291us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember | 1,175us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |
| Interest Income Recognized                                                                                                     |                                                                                                                                                                                                             | 9us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember |   |
| Commercial and farmland                                                                                                        |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 95,856us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      | 128,073us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 58,173us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Recorded Investment                                                                                                            | 68,187us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember          | 83,033us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 43,204us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Average Recorded Investment                                                                                                    | 70,844us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember   | 84,057us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 46,788us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Interest Income Recognized                                                                                                     | 3,389us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  | 1,663us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 1,211us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Unpaid Principal Balance                                                                                                       | 7,159us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember         | 74us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 5,579us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Recorded Investment                                                                                                            | 4,971us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember             | 71us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 5,182us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Related Allowance                                                                                                              | 1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                   | 6us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 2,460us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Average Recorded Investment                                                                                                    | 4,999us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      | 76us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 5,239us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Interest Income Recognized                                                                                                     | 24us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember       |    | 95us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Residential                                                                                                                    |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 10,591us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember        | 6,746us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 8,820us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Recorded Investment                                                                                                            | 6,839us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember             | 3,910us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 6,215us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Average Recorded Investment                                                                                                    | 6,987us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember      | 4,236us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 7,129us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Interest Income Recognized                                                                                                     | 119us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember      | 75us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 83us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Unpaid Principal Balance                                                                                                       | 1,001us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember           |   | 1,722us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Recorded Investment                                                                                                            | 998us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                 |   | 1,451us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Related Allowance                                                                                                              | 194us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                       |   | 50us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Average Recorded Investment                                                                                                    | 1,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember        |   | 1,458us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Interest Income Recognized                                                                                                     |                                                                                                                                                                                                        |   | 75us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Home equity                                                                                                                    |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 3,590us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                    | 3,299us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember | 4,199us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Recorded Investment                                                                                                            | 398us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                          | 112us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember | 1,006us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Average Recorded Investment                                                                                                    | 402us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                   | 225us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember | 1,022us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Interest Income Recognized                                                                                                     |                                                                                                                                                                                                        |    | 13us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember |
| Other loans                                                                                                                    |                                                                                                                                                                                                             |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                |                                                                                                                                                                                                             |   |   |
| Unpaid Principal Balance                                                                                                       | 30us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                          | 454us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember | 83us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember |
| Recorded Investment                                                                                                            |                                                                                                                                                                                                        | 172us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember | 14us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember |
| Average Recorded Investment                                                                                                    |                                                                                                                                                                                                        | 181us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember | 18us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember |
| Interest Income Recognized                                                                                                     |                                                                                                                                                                                                        | $ 1us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember | $ 1us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember |

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| - 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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| - Definition
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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| - Definition
Amount of allowance for credit losses related to recorded investment.
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| - Definition
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| - Definition
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 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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Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
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