| **Loans and Allowance - Composition of Loan Portfolio by Loan Class (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                           | Dec. 31, 2013                                                                                                                                                                           | Dec. 31, 2012                                                                                                                                                         | Dec. 31, 2011                                                                                                                                                               |
|--------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | $ 3,924,865us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                          | $ 3,632,409us-gaap_LoansAndLeasesReceivableGrossCarryingAmount                                                                                                                          |                                                                                                                                                                  |                                                                                                                                                                        |
| Allowance for loan losses                                                                                                      | (63,964)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                       | (67,870)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                       | (69,366)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     | (70,898)us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                           |
| Net loans                                                                                                                      | 3,860,901us-gaap_LoansAndLeasesReceivableNetReportedAmount                                                                                                                              | 3,564,539us-gaap_LoansAndLeasesReceivableNetReportedAmount                                                                                                                              |                                                                                                                                                                  |                                                                                                                                                                        |
| Commercial and industrial loans                                                                                                |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 896,688us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember      | 761,705us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember      |                                                                                                                                                                  |                                                                                                                                                                        |
| Agricultural production financing and other loans to farmers                                                                   |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 104,927us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember             | 114,348us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember             |                                                                                                                                                                  |                                                                                                                                                                        |
| Construction                                                                                                                   |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 207,221us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember              | 177,082us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember              |                                                                                                                                                                  |                                                                                                                                                                        |
| Commercial and farmland                                                                                                        |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 1,672,661us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      | 1,611,809us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember      |                                                                                                                                                                  |                                                                                                                                                                        |
| Residential                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 647,315us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember          | 616,385us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember          |                                                                                                                                                                  |                                                                                                                                                                        |
| Allowance for loan losses                                                                                                      | (13,152)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                   | (15,077)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                   | (14,157)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | (12,343)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |
| Home equity                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 286,529us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                     | 255,223us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                     |                                                                                                                                                                  |                                                                                                                                                                        |
| Individuals' loans for household and other personal expenditures                                                               |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 73,400us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                    | 69,783us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Allowance for loan losses                                                                                                      | (2,658)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                             | (2,515)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                             | (2,593)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           | (2,902)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 |
| Lease financing receivables, net of unearned income                                                                            |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | 1,106us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember | 1,545us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |                                                                                                                                                                  |                                                                                                                                                                        |
| Allowance for loan losses                                                                                                      | (3)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember             |                                                                                                                                                                               |                                                                                                                                                             | (3)us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |
| Other loans                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| **Loans:**                                                                                                                     |                                                                                                                                                                                    |                                                                                                                                                                                    |                                                                                                                                                                  |                                                                                                                                                                        |
| Loans                                                                                                                          | $ 35,018us-gaap_LoansAndLeasesReceivableGrossCarryingAmount/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_OtherLoansMember                       | $ 24,529us-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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
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