| **Accounting For Certain Loans Acquired In a Purchase - Carrying Amount of Loans Included in Balance Sheet (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                                                                                    | Dec. 31, 2013                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | $ 698,457us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance                                                                                                                                                                                                    | $ 690,218us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance                                                                                                                                                                                                    |
| Carrying Amount                                                                                                                                                        | 658,471frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross                                                                                                                                                                                                     | 636,182frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross                                                                                                                                                                                                     |
| Allowance                                                                                                                                                              | 650us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses                                                                                                                                                                                                   | 229us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses                                                                                                                                                                                                   |
| Carrying Amount                                                                                                                                                        | 657,821us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet                                                                                                                                                                                                    | 635,953us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet                                                                                                                                                                                                    |
| Merger with Community Bancshares, Inc.                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 142,989us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                       |                                                                                                                                                                                                                                                                                             |
| Carrying Amount                                                                                                                                                        | 134,198frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                      |                                                                                                                                                                                                                                                                                             |
| Allowance                                                                                                                                                              |                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                             |
| Carrying Amount                                                                                                                                                        | 134,198us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                                     |                                                                                                                                                                                                                                                                                             |
| Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 509,660us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                        | 629,184us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                        |
| Carrying Amount                                                                                                                                                        | 484,949frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                       | 585,913frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                       |
| Allowance                                                                                                                                                              | 650us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                     |                                                                                                                                                                                                                                                                                             |
| Carrying Amount                                                                                                                                                        | 484,299us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                      | 585,913us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                                                      |
| SCB Bank                                                                                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 45,808us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                  | 61,034us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                  |
| Carrying Amount                                                                                                                                                        | 39,324frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                 | 50,269frme_CertainLoansAcquiredinTransferNotAccountedforasDebtSecuritiesCarryingAmountGross/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                 |
| Allowance                                                                                                                                                              |                                                                                                                                                                                                                                                                                        | 229us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAllowanceForLoanLosses/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                              |
| Carrying Amount                                                                                                                                                        | 39,324us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                | 50,040us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember                                                                                                                                                |
| Commercial and industrial loans                                                                                                                                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 79,124us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                                                               | 89,487us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                                                               |
| Commercial and industrial loans | Merger with Community Bancshares, Inc.                                                                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 8,168us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |                                                                                                                                                                                                                                                                                             |
| Commercial and industrial loans | Merger with CFS Bancorp, Inc. (Citizens)                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 64,897us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 81,303us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Commercial and industrial loans | SCB Bank                                                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 6,059us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           | 8,184us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                           |
| Agriculture production financing and other loans to farmers                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,993us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                       | 1,161us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                                                                       |
| Agriculture production financing and other loans to farmers | Merger with Community Bancshares, Inc.                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,100us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember        |                                                                                                                                                                                                                                                                                             |
| Agriculture production financing and other loans to farmers | Merger with CFS Bancorp, Inc. (Citizens)                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                        |
| Agriculture production financing and other loans to farmers | SCB Bank                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 893us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                    | 1,161us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember                                  |
| Construction                                                                                                                                                           |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 28,176us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                                                                       | 17,962us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                                                                       |
| Construction | Merger with Community Bancshares, Inc.                                                                                                                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 19,063us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember        |                                                                                                                                                                                                                                                                                             |
| Construction | Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 9,113us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember          | 17,962us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember         |
| Construction | SCB Bank                                                                                                                                                |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                        |
| Commercial and farmland                                                                                                                                                |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 341,195us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                                                | 335,049us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                                                |
| Commercial and farmland | Merger with Community Bancshares, Inc.                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 74,600us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  |                                                                                                                                                                                                                                                                                             |
| Commercial and farmland | Merger with CFS Bancorp, Inc. (Citizens)                                                                                                     |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 251,002us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  | 311,631us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  |
| Commercial and farmland | SCB Bank                                                                                                                                     |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 15,593us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                            | 23,418us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                            |
| Residential                                                                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 180,643us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                                                  | 176,113us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                                                  |
| Residential | Merger with Community Bancshares, Inc.                                                                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 28,863us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    |                                                                                                                                                                                                                                                                                             |
| Residential | Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 144,396us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | 166,754us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    |
| Residential | SCB Bank                                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 7,384us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               | 9,359us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                               |
| Home equity                                                                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 64,883us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                                                                              | 67,278us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                                                                              |
| Home equity | Merger with Community Bancshares, Inc.                                                                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 9,881us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                |                                                                                                                                                                                                                                                                                             |
| Home equity | Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 39,244us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                | 49,042us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                |
| Home equity | SCB Bank                                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 15,758us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                         | 18,236us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                                         |
| Individuals' loans for household and other personal expenditures                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 2,357us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                                             | 2,629us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                                             |
| Individuals' loans for household and other personal expenditures | Merger with Community Bancshares, Inc.                                                              |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 1,314us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember              |                                                                                                                                                                                                                                                                                             |
| Individuals' loans for household and other personal expenditures | Merger with CFS Bancorp, Inc. (Citizens)                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 922us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 | 2,360us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember               |
| Individuals' loans for household and other personal expenditures | SCB Bank                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 121us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                          | 269us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                          |
| Other loans                                                                                                                                                            |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 86us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                     | 539us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                                                                                    |
| Other loans | Merger with Community Bancshares, Inc.                                                                                                                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                             |
| Other loans | Merger with CFS Bancorp, Inc. (Citizens)                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         | 86us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                       | 132us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                      |
| Other loans | SCB Bank                                                                                                                                                 |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                                                       |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                             |
| Loans Acquired                                                                                                                                                         |                                                                                                                                                                                                                                                                                        | $ 407us-gaap_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesOutstandingBalance/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                                             |

| 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:** |