| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                  | Dec. 31, 2014                                                                                                                                                                                                    |
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| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | $ 3,904,616us-gaap_FinancingReceivableRecordedInvestmentCurrent                                                                                                                                                | $ 3,846,157us-gaap_FinancingReceivableRecordedInvestmentCurrent                                                                                                                                                  |
| 30-59 Days Past Due                                                                                                                | 12,312us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue                                                                                                                                           | 22,107us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue                                                                                                                                             |
| 60-89 Days Past Due                                                                                                                | 2,628us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue                                                                                                                                            | 3,149us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue                                                                                                                                              |
| Loans 90 Days And Accruing                                                                                                         | 1,655frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing                                                                                                                       | 4,663frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing                                                                                                                         |
| Non-Accrual                                                                                                                        | 44,321us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                            | 48,789us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                              |
| Total Past Due & Non-Accrual                                                                                                   | 60,916us-gaap_FinancingReceivableRecordedInvestmentPastDue                                                                                                                                                     | 78,708us-gaap_FinancingReceivableRecordedInvestmentPastDue                                                                                                                                                       |
| Total                                                                                                                              | 3,965,532us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                   | 3,924,865us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                     |
| Commercial and industrial loans                                                                                                    |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 927,244us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                            | 882,596us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                              |
| 30-59 Days Past Due                                                                                                                | 2,057us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                    | 4,006us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                      |
| 60-89 Days Past Due                                                                                                                | 1,166us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                    | 53us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                         |
| Loans 90 Days And Accruing                                                                                                         | 221frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 2,985frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Non-Accrual                                                                                                                        | 8,249us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                     | 7,048us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                       |
| Total Past Due & Non-Accrual                                                                                                   | 11,693us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                             | 14,092us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                               |
| Total                                                                                                                              | 938,937us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                             | 896,688us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                               |
| Agricultural production financing and other loans to farmers                                                                       |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 93,349us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                    | 98,236us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                      |
| 30-59 Days Past Due                                                                                                                | 325us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                             | 891us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                               |
| 60-89 Days Past Due                                                                                                                |                                                                                                                                                                                                      |                                                                                                                                                                                                             |
| Non-Accrual                                                                                                                        | 1,978us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                            | 5,800us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                              |
| Total Past Due & Non-Accrual                                                                                                   | 2,303us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                     | 6,691us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                       |
| Total                                                                                                                              | 95,652us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                     | 104,927us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                      |
| Real estate loans - Construction                                                                                                   |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 235,077us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                    | 204,683us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                      |
| 30-59 Days Past Due                                                                                                                | 138us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                              | 1,017us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                              |
| 60-89 Days Past Due                                                                                                                |                                                                                                                                                                                                           | 82us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                 |
| Loans 90 Days And Accruing                                                                                                         | 236frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember         |                                                                                                                                                                                                             |
| Non-Accrual                                                                                                                        | 1,585us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                             | 1,439us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                               |
| Total Past Due & Non-Accrual                                                                                                   | 1,959us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                      | 2,538us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                        |
| Total                                                                                                                              | 237,036us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                     | 207,221us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                       |
| Real estate loans - Commercial and farmland                                                                                        |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 1,623,656us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                            | 1,642,016us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                              |
| 30-59 Days Past Due                                                                                                                | 5,261us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                      | 9,846us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                        |
| 60-89 Days Past Due                                                                                                                | 305us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                        | 778us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                          |
| Loans 90 Days And Accruing                                                                                                         | 265frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember   | 671frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember     |
| Non-Accrual                                                                                                                        | 16,931us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                      | 19,350us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                        |
| Total Past Due & Non-Accrual                                                                                                   | 22,762us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                               | 30,645us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                 |
| Total                                                                                                                              | 1,646,418us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                             | 1,672,661us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                               |
| Real estate loans - Residential                                                                                                    |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 623,394us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                | 626,821us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                  |
| 30-59 Days Past Due                                                                                                                | 2,446us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                        | 4,876us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          |
| 60-89 Days Past Due                                                                                                                | 761us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          | 1,831us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          |
| Loans 90 Days And Accruing                                                                                                         | 536frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember     | 854frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |
| Non-Accrual                                                                                                                        | 13,314us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                        | 12,933us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          |
| Total Past Due & Non-Accrual                                                                                                   | 17,057us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                 | 20,494us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                   |
| Total                                                                                                                              | 640,451us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                 | 647,315us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                   |
| Real estate loans - Home equity                                                                                                    |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 282,344us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                           | 282,828us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                             |
| 30-59 Days Past Due                                                                                                                | 1,734us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                   | 1,213us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                     |
| 60-89 Days Past Due                                                                                                                | 316us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                     | 352us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                       |
| Loans 90 Days And Accruing                                                                                                         | 379frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                | 148frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                  |
| Non-Accrual                                                                                                                        | 2,141us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                    | 1,988us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                      |
| Total Past Due & Non-Accrual                                                                                                   | 4,570us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                             | 3,701us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                               |
| Total                                                                                                                              | 286,914us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                            | 286,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                              |
| Individuals' loans for household and other personal expenditures                                                                   |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 69,829us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                          | 72,853us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                            |
| 30-59 Days Past Due                                                                                                                | 173us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                   | 258us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                     |
| 60-89 Days Past Due                                                                                                                | 80us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                    | 53us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                      |
| Loans 90 Days And Accruing                                                                                                         | 18frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember               | 5frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                  |
| Non-Accrual                                                                                                                        | 123us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                    | 231us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                      |
| Total Past Due & Non-Accrual                                                                                                   | 394us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                             | 547us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                               |
| Total                                                                                                                              | 70,223us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                           | 73,400us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                             |
| Lease financing receivables, net of unearned income                                                                                |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 675us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                         | 1,106us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                         |
| 30-59 Days Past Due                                                                                                                | 178us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember               |                                                                                                                                                                                                             |
| Total Past Due & Non-Accrual                                                                                                   | 178us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                         |                                                                                                                                                                                                             |
| Total                                                                                                                              | 853us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                          | 1,106us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                          |
| Other loans                                                                                                                        |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                               |                                                                                                                                                                                                           |                                                                                                                                                                                                             |
| Current                                                                                                                            | 49,048us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                               | 35,018us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                 |
| 30-59 Days Past Due                                                                                                                |                                                                                                                                                                                                      |                                                                                                                                                                                                             |
| Non-Accrual                                                                                                                        |                                                                                                                                                                                                           |                                                                                                                                                                                                        |
| Total Past Due & Non-Accrual                                                                                                   |                                                                                                                                                                                                      |                                                                                                                                                                                                        |
| Total                                                                                                                              | $ 49,048us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                              | $ 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                |

| X                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, More Than Ninety Days Past Due and Still Accruing
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are less than 60 days past due but more than 29 days past due.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are less than 90 days past due but more than 59 days past due.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are current.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentCurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_OtherLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_FinanceLeasesFinancingReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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