| **Accounting for Certain Loans Acquired in a Purchase - Carrying Amount of Loans Included in Balance Sheet (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                                                                    | Dec. 31, 2014                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | $ 652,186us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance                                                                                                                                                                                                    | $ 698,457us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance                                                                                                                                                                                                    |
| Carrying Amount                                                                                                                                                        | 614,843frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross                                                                                                                                                                                                     | 658,471frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross                                                                                                                                                                                                     |
| Allowance                                                                                                                                                              | 480us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses                                                                                                                                                                                                   | 650us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses                                                                                                                                                                                                   |
| Carrying Amount Net of Allowance                                                                                                                                       | 614,363us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet                                                                                                                                                                                                    | 657,821us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet                                                                                                                                                                                                    |
| Commercial and industrial loans                                                                                                                                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 70,753us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                                                               | 79,124us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                                                               |
| Agricultural production financing and other loans to farmers                                                                                                           |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,614us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                       | 1,993us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                       |
| Real estate loans - Construction                                                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 27,303us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                                                                       | 28,176us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                                                                       |
| Real estate loans - Commercial and farmland                                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 316,523us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                                                | 341,195us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                                                |
| Real estate loans - Residential                                                                                                                                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 172,373us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                                                  | 180,643us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                                                  |
| Real estate loans - Home equity                                                                                                                                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 61,606us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                                                                              | 64,883us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                                                                              |
| Individuals' loans for household and other personal expenditures                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,930us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                                             | 2,357us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                                             |
| Other loans                                                                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 84us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                     | 86us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                     |
| Merger with Community Bancshares, Inc.                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 135,383us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                       | 142,989us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                       |
| Carrying Amount                                                                                                                                                        | 126,996frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                      | 134,198frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                      |
| Allowance                                                                                                                                                              | 195us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                    |                                                                                                                                                                                                                                                                                        |
| Carrying Amount Net of Allowance                                                                                                                                       | 126,801us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                     | 134,198us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                     |
| Merger with Community Bancshares, Inc. | Commercial and industrial loans                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 6,828us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 8,168us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Merger with Community Bancshares, Inc. | Agricultural production financing and other loans to farmers                                                                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 737us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember          | 1,100us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember        |
| Merger with Community Bancshares, Inc. | Real estate loans - Construction                                                                                              |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 19,969us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember        | 19,063us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember        |
| Merger with Community Bancshares, Inc. | Real estate loans - Commercial and farmland                                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 69,421us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  | 74,600us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  |
| Merger with Community Bancshares, Inc. | Real estate loans - Residential                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 28,048us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | 28,863us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    |
| Merger with Community Bancshares, Inc. | Real estate loans - Home equity                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 9,324us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                | 9,881us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                |
| Merger with Community Bancshares, Inc. | Individuals' loans for household and other personal expenditures                                                              |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,056us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember              | 1,314us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember              |
| Merger with Community Bancshares, Inc. | Other loans                                                                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                        |
| Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 473,781us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                        | 509,660us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                        |
| Carrying Amount                                                                                                                                                        | 450,867frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                       | 484,949frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                       |
| Allowance                                                                                                                                                              | 285us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                     | 650us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                     |
| Carrying Amount Net of Allowance                                                                                                                                       | 450,582us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                      | 484,299us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                      |
| Merger with CFS Bancorp, Inc. (Citizens) | Commercial and industrial loans                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 58,252us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 64,897us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Merger with CFS Bancorp, Inc. (Citizens) | Real estate loans - Construction                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 7,334us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 9,113us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          |
| Merger with CFS Bancorp, Inc. (Citizens) | Real estate loans - Commercial and farmland                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 232,436us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  | 251,002us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  |
| Merger with CFS Bancorp, Inc. (Citizens) | Real estate loans - Residential                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 137,457us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | 144,396us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    |
| Merger with CFS Bancorp, Inc. (Citizens) | Real estate loans - Home equity                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 37,439us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                | 39,244us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                |
| Merger with CFS Bancorp, Inc. (Citizens) | Individuals' loans for household and other personal expenditures                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 779us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 | 922us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 |
| Merger with CFS Bancorp, Inc. (Citizens) | Other loans                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 84us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                       | 86us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                       |
| SCB Bank                                                                                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 43,022us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                  | 45,808us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                  |
| Carrying Amount                                                                                                                                                        | 36,980frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                 | 39,324frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                 |
| Allowance                                                                                                                                                              |                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                        |
| Carrying Amount Net of Allowance                                                                                                                                       | 36,980us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                | 39,324us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                |
| SCB Bank | Commercial and industrial loans                                                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 5,673us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           | 6,059us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           |
| SCB Bank | Agricultural production financing and other loans to farmers                                                                                                |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 877us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                    | 893us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                    |
| SCB Bank | Real estate loans - Commercial and farmland                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 14,666us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                            | 15,593us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                            |
| SCB Bank | Real estate loans - Residential                                                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 6,868us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               | 7,384us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               |
| SCB Bank | Real estate loans - Home equity                                                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 14,843us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                         | 15,758us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                         |
| SCB Bank | Individuals' loans for household and other personal expenditures                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 95us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                           | 121us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                          |
| SCB Bank | Other loans                                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                        |

| X                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount Gross
+ References
No definition available.
+ Details**Name:**frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor.  This relates to loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected.  This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses).  This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=frme_MergerwithCFSBancorpInc.CitizensMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=frme_MergerwithCommunityBancsharesInc.Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=frme_ScbBankMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_OtherLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_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:** |