| **Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                                                           | Dec. 31, 2013                                                                                                                                                                                                                                                            |
|-----------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | $ 3,924,865us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                                                                          | $ 3,632,409us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                                                                                                           |
| Commercial and industrial loans                                                                                                   |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 104,927us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                             | 114,348us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                              |
| Construction                                                                                                                      |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| 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 [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                         | 24,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                          |
| Commercial Pass                                                                                                                   |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 2,849,876us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                                                                                                                           | 2,635,448us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                                                                                                                            |
| Commercial Pass | Commercial and industrial loans                                                                                 |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 823,732us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                     | 708,835us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                      |
| Commercial Pass | Agricultural production financing and other loans to farmers                                                    |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 96,155us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                             | 114,318us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                             |
| Commercial Pass | Construction                                                                                                    |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 185,394us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                             | 162,976us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                              |
| Commercial Pass | Commercial and farmland                                                                                         |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 1,552,781us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                     | 1,473,714us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                      |
| Commercial Pass | Residential                                                                                                     |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 149,430us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                         | 143,657us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                          |
| Commercial Pass | Home equity                                                                                                     |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 6,368us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                      | 6,194us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                       |
| Commercial Pass | Finance Leases                                                                                                  |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 998us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                  | 1,420us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                 |
| Commercial Pass | Other loans                                                                                                     |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                        | 24,334us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialPassMember                                         |
| Commercial Special Mention                                                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 64,631us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                                                                                                                                    | 72,407us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                                                                                                                                     |
| Commercial Special Mention | Commercial and industrial loans                                                                      |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 24,455us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember            | 11,332us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember             |
| Commercial Special Mention | Agricultural production financing and other loans to farmers                                         |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 1,195us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                    |                                                                                                                                                                                                                                                                     |
| Commercial Special Mention | Construction                                                                                         |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 3,164us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                     | 1,132us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                      |
| Commercial Special Mention | Commercial and farmland                                                                              |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 29,484us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember              | 57,676us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember               |
| Commercial Special Mention | Residential                                                                                          |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 6,321us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                 | 2,232us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                  |
| Commercial Special Mention | Home equity                                                                                          |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 12us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                               | 35us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSpecialMentionMember                                |
| Commercial Substandard                                                                                                            |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 160,608us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                                                                                                                                      | 146,254us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                                                                                                                                       |
| Commercial Substandard | Commercial and industrial loans                                                                          |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 48,226us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember               | 41,013us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                |
| Commercial Substandard | Agricultural production financing and other loans to farmers                                             |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 7,577us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                       | 30us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                           |
| Commercial Substandard | Construction                                                                                             |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 2,928us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                        | 12,029us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                        |
| Commercial Substandard | Commercial and farmland                                                                                  |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 90,161us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                 | 80,184us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                  |
| Commercial Substandard | Residential                                                                                              |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 10,918us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                   | 11,494us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                    |
| Commercial Substandard | Home equity                                                                                              |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 690us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                                 | 1,184us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                                |
| Commercial Substandard | Finance Leases                                                                                           |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 108us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember           | 125us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember            |
| Commercial Substandard | Other loans                                                                                              |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             |                                                                                                                                                                                                                                                                    | 195us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialSubstandardMember                                     |
| Commercial Doubtful                                                                                                               |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 275us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialDoubtfulMember                                                                                                                                             | 661us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialDoubtfulMember                                                                                                                                              |
| Commercial Doubtful | Commercial and industrial loans                                                                             |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 275us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialDoubtfulMember                     | 525us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialDoubtfulMember                      |
| Commercial Doubtful | Residential                                                                                                 |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             |                                                                                                                                                                                                                                                                    | 136us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_CommercialDoubtfulMember                          |
| Consumer Performing                                                                                                               |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 837,296us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember                                                                                                                      | 764,261us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember                                                                                                                       |
| Consumer Performing | Construction                                                                                                |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 15,588us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember       |                                                                                                                                                                                                                                                                     |
| Consumer Performing | Residential                                                                                                 |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 470,972us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember  | 448,494us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember   |
| Consumer Performing | Home equity                                                                                                 |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 277,571us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember             | 246,101us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember              |
| Consumer Performing | Consumer                                                                                                    |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 73,165us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember            | 69,666us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerPerformingFinancingReceivableMember             |
| Consumer Non Performing                                                                                                           |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 12,179us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember                                                                                                                    | 13,378us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember                                                                                                                     |
| Consumer Non Performing | Construction                                                                                            |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 147us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember       | 945us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember        |
| Consumer Non Performing | Commercial and farmland                                                                                 |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 235us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember | 235us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember  |
| Consumer Non Performing | Residential                                                                                             |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 9,674us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember | 10,372us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember |
| Consumer Non Performing | Home equity                                                                                             |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | 1,888us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember            | 1,709us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember             |
| Consumer Non Performing | Consumer                                                                                                |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| **Financing Receivable, Recorded Investment [Line Items]**                                                                        |                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                     |
| Loans                                                                                                                             | $ 235us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember          | $ 117us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember/ us-gaap_InternalCreditAssessmentAxis= frme_ConsumerNonperformingFinancingReceivableMember           |

| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentLineItems**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                                                                                                                                                                                                           |
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| 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                                                                                                                                                                                              |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_CommercialDoubtfulMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_CommercialPassMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
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| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_CommercialSpecialMentionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
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| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_CommercialSubstandardMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_ConsumerNonperformingFinancingReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_ConsumerPerformingFinancingReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |