| **Loans and Allowance - Composition of Loan Portfolio by Loan Class (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                         | Dec. 31, 2014                                                                                                                                                                           | Mar. 31, 2014                                                                                                                                                         | Dec. 31, 2013                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | $ 3,965,532us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                        | $ 3,924,865us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                          |                                                                                                                                                                  |                                                                                                                                                                  |
| Allowance for loan losses                                                                                                       | (62,801)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                     | (63,964)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                       | (69,583)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     | (67,870)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     |
| Net Loans                                                                                                                       | 3,902,731us-gaap_LoansAndLeasesReceivableNetReportedAmount                                                                                                                            | 3,860,901us-gaap_LoansAndLeasesReceivableNetReportedAmount                                                                                                                              |                                                                                                                                                                  |                                                                                                                                                                  |
| Commercial and industrial loans                                                                                                 |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 938,937us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember    | 896,688us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember      |                                                                                                                                                                  |                                                                                                                                                                  |
| Agricultural production financing and other loans to farmers                                                                    |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 95,652us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember            | 104,927us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember             |                                                                                                                                                                  |                                                                                                                                                                  |
| Real estate loans - Construction                                                                                                |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 237,036us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember            | 207,221us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember              |                                                                                                                                                                  |                                                                                                                                                                  |
| Real estate loans - Commercial and farmland                                                                                     |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 1,646,418us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember    | 1,672,661us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      |                                                                                                                                                                  |                                                                                                                                                                  |
| Real estate loans - Residential                                                                                                 |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 640,451us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember        | 647,315us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember          |                                                                                                                                                                  |                                                                                                                                                                  |
| Allowance for loan losses                                                                                                       | (13,269)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                 | (13,152)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                   | (13,908)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | (15,077)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Real estate loans - Home Equity                                                                                                 |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 286,914us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                   | 286,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                     |                                                                                                                                                                  |                                                                                                                                                                  |
| Individuals' loans for household and other personal expenditures                                                                |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 70,223us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                  | 73,400us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Allowance for loan losses                                                                                                       | (3,138)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                           | (2,658)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                             | (2,410)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           | (2,515)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           |
| Lease financing receivables, net of unearned income                                                                             |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | 853us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember | 1,106us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |                                                                                                                                                                  |                                                                                                                                                                  |
| Allowance for loan losses                                                                                                       | (4)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember           | (3)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember             |                                                                                                                                                             |                                                                                                                                                             |
| Other loans                                                                                                                     |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| **Loans:**                                                                                                                      |                                                                                                                                                                                  |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                  |
| Loans                                                                                                                           | $ 49,048us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                     | $ 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                       |                                                                                                                                                                  |                                                                                                                                                                  |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
+ References
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| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.
+ References
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