| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                    | Dec. 31, 2013                                                                                                                                                                                                |
|-----------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | $ 3,846,157us-gaap_FinancingReceivableRecordedInvestmentCurrent                                                                                                                                                  | $ 3,549,125us-gaap_FinancingReceivableRecordedInvestmentCurrent                                                                                                                                              |
| 30-59 Days Past Due                                                                                                               | 22,107us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue                                                                                                                                             | 16,864us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue                                                                                                                                         |
| 60-89 Days Past Due                                                                                                               | 3,149us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue                                                                                                                                              | 8,668us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue                                                                                                                                          |
| Loans 90 Days And Accruing                                                                                                        | 4,663frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing                                                                                                                         | 1,350frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing                                                                                                                     |
| Non-Accrual                                                                                                                       | 48,789us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                              | 56,402us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                          |
| Total Past Due & Non-Accrual                                                                                                  | 78,708us-gaap_FinancingReceivableRecordedInvestmentPastDue                                                                                                                                                       | 83,284us-gaap_FinancingReceivableRecordedInvestmentPastDue                                                                                                                                                   |
| Total Loans                                                                                                                       | 3,924,865us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                     | 3,632,409us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                 |
| Commercial and industrial loans                                                                                                   |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 882,596us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                              | 749,020us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                          |
| 30-59 Days Past Due                                                                                                               | 4,006us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                      | 2,628us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                  |
| 60-89 Days Past Due                                                                                                               | 53us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                         | 774us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                    |
| Loans 90 Days And Accruing                                                                                                        | 2,985frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |                                                                                                                                                                                                    |
| Non-Accrual                                                                                                                       | 7,048us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                       | 9,283us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                   |
| Total Past Due & Non-Accrual                                                                                                  | 14,092us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                               | 12,685us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           |
| Total Loans                                                                                                                       | 896,688us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                               | 761,705us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           |
| Agricultural production financing and other loans to farmers                                                                      |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 98,236us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                      | 114,305us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                 |
| 30-59 Days Past Due                                                                                                               | 891us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                               | 13us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                            |
| 60-89 Days Past Due                                                                                                               |                                                                                                                                                                                                             |                                                                                                                                                                                                    |
| Loans 90 Days And Accruing                                                                                                        |                                                                                                                                                                                                             |                                                                                                                                                                                                    |
| Non-Accrual                                                                                                                       | 5,800us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                              | 30us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                             |
| Total Past Due & Non-Accrual                                                                                                  | 6,691us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                       | 43us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                      |
| Total Loans                                                                                                                       | 104,927us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                      | 114,348us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                  |
| Construction                                                                                                                      |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 204,683us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                      | 171,046us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                  |
| 30-59 Days Past Due                                                                                                               | 1,017us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                              | 1,058us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                          |
| 60-89 Days Past Due                                                                                                               | 82us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                 |                                                                                                                                                                                                         |
| Non-Accrual                                                                                                                       | 1,439us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                               | 4,978us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                           |
| Total Past Due & Non-Accrual                                                                                                  | 2,538us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                        | 6,036us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                    |
| Total Loans                                                                                                                       | 207,221us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                       | 177,082us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                   |
| Commercial and farmland                                                                                                           |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 1,642,016us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                              | 1,573,403us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                          |
| 30-59 Days Past Due                                                                                                               | 9,846us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                        | 3,807us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                    |
| 60-89 Days Past Due                                                                                                               | 778us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                          | 5,801us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                    |
| Loans 90 Days And Accruing                                                                                                        | 671frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember     | 703frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Non-Accrual                                                                                                                       | 19,350us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                        | 28,095us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                    |
| Total Past Due & Non-Accrual                                                                                                  | 30,645us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                 | 38,406us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                             |
| Total Loans                                                                                                                       | 1,672,661us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                               | 1,611,809us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                           |
| Residential                                                                                                                       |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 626,821us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                  | 595,192us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                              |
| 30-59 Days Past Due                                                                                                               | 4,876us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          | 7,156us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                      |
| 60-89 Days Past Due                                                                                                               | 1,831us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          | 1,475us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                      |
| Loans 90 Days And Accruing                                                                                                        | 854frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       | 494frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember   |
| Non-Accrual                                                                                                                       | 12,933us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          | 12,068us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                      |
| Total Past Due & Non-Accrual                                                                                                  | 20,494us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                   | 21,193us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               |
| Total Loans                                                                                                                       | 647,315us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                   | 616,385us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               |
| Home equity                                                                                                                       |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 282,828us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                             | 251,188us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                         |
| 30-59 Days Past Due                                                                                                               | 1,213us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                     | 1,652us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                 |
| 60-89 Days Past Due                                                                                                               | 352us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                       | 563us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                   |
| Loans 90 Days And Accruing                                                                                                        | 148frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                  | 153frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember              |
| Non-Accrual                                                                                                                       | 1,988us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                      | 1,667us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                  |
| Total Past Due & Non-Accrual                                                                                                  | 3,701us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                               | 4,035us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                           |
| Total Loans                                                                                                                       | 286,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                              | 255,223us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                          |
| Consumer                                                                                                                          |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 72,853us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                            | 69,061us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                        |
| 30-59 Days Past Due                                                                                                               | 258us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                     | 550us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                 |
| 60-89 Days Past Due                                                                                                               | 53us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                      | 55us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                  |
| Loans 90 Days And Accruing                                                                                                        | 5frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                  |                                                                                                                                                                                                         |
| Non-Accrual                                                                                                                       | 231us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                      | 117us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                  |
| Total Past Due & Non-Accrual                                                                                                  | 547us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                               | 722us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                           |
| Total Loans                                                                                                                       | 73,400us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                             | 69,783us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                         |
| Finance Leases                                                                                                                    |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 1,106us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                         | 1,545us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                     |
| Non-Accrual                                                                                                                       |                                                                                                                                                                                                             |                                                                                                                                                                                                    |
| Total Past Due & Non-Accrual                                                                                                  |                                                                                                                                                                                                             |                                                                                                                                                                                                    |
| Total Loans                                                                                                                       | 1,106us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                          | 1,545us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                      |
| Other loans                                                                                                                       |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                              |                                                                                                                                                                                                             |                                                                                                                                                                                                         |
| Current                                                                                                                           | 35,018us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                 | 24,365us-gaap_FinancingReceivableRecordedInvestmentCurrent/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                             |
| Non-Accrual                                                                                                                       |                                                                                                                                                                                                        | 164us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                       |
| Total Past Due & Non-Accrual                                                                                                  |                                                                                                                                                                                                        | 164us-gaap_FinancingReceivableRecordedInvestmentPastDue/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                |
| Total Loans                                                                                                                       | $ 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                | $ 24,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                            |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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| 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
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| 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
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| X                                                                                                                                                                                         |
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| 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                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| 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                                                                                                                                                                                              |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
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