| **Loans and Allowance - Allowance for Credit Losses and Loan Portfolio by Loan Segment (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                         | Dec. 31, 2013                                                                                                                                                                                                   | Dec. 31, 2012                                                                                                                                                       | Dec. 31, 2011                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Individually evaluated for impairment                                                                                        | $ 2,119us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1                                                                                                                         | $ 1,354us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1                                                                                                                   |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 61,195us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment                                                                                                                           | 66,287us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment                                                                                                                     |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Loans Acquired with Deteriorated Credit Quality                                                                              | 650us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality                                                                                                                           | 229us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality                                                                                                                     |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 63,964us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                       | 67,870us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                 | 69,366us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     | 70,898us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                           |
| Loan balances - Individually evaluated for impairment                                                                                             | 44,093us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment                                                                                                                                                   | 42,067us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment                                                                                                                                             |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Collectively evaluated for impairment                                                                                             | 3,807,707us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment                                                                                                                                                | 3,511,501us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment                                                                                                                                          |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Loans Acquired with Deteriorated Credit Quality                                                                                   | 73,065us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality                                                                                                                                                | 78,841us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality                                                                                                                                          |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | 3,924,865frme_LoansAndLeasesReceivable                                                                                                                                                                                | 3,632,409frme_LoansAndLeasesReceivable                                                                                                                                                                          |                                                                                                                                                                |                                                                                                                                                                      |
| Commercial                                                                                                                                        |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Individually evaluated for impairment                                                                                        | 1,455us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              | 585us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember          |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 27,369us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              | 26,493us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember        |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Loans Acquired with Deteriorated Credit Quality                                                                              |                                                                                                                                                                                                             | 98us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember         |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 28,824us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                          | 27,176us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                    | 25,913us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember        | 17,731us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              |
| Loan balances - Individually evaluated for impairment                                                                                             | 16,108us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                      | 10,240us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Collectively evaluated for impairment                                                                                             | 1,011,122us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                   | 882,794us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                               |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Loans Acquired with Deteriorated Credit Quality                                                                                   | 9,403us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                    | 7,548us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                              |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | 1,036,633frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                                   | 900,582frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                               |                                                                                                                                                                |                                                                                                                                                                      |
| Commercial Real Estate                                                                                                                            |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Individually evaluated for impairment                                                                                        | 470us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember          | 763us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember    |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 18,207us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        | 22,208us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember  |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Loans Acquired with Deteriorated Credit Quality                                                                              | 650us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        | 131us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember  |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 19,327us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                    | 23,102us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                              | 26,703us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember  | 37,919us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        |
| Loan balances - Individually evaluated for impairment                                                                                             | 23,963us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                | 29,007us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                          |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Collectively evaluated for impairment                                                                                             | 1,796,797us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             | 1,690,285us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                       |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Loans Acquired with Deteriorated Credit Quality                                                                                   | 59,122us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             | 69,599us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                       |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | 1,879,882frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                             | 1,788,891frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                       |                                                                                                                                                                |                                                                                                                                                                      |
| Consumer                                                                                                                                          |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 2,658us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 | 2,515us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 2,658us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                             | 2,515us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                       | 2,593us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           | 2,902us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 |
| Loan balances - Collectively evaluated for impairment                                                                                             | 73,400us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                        | 69,783us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                  |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | 73,400frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                        | 69,783frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                  |                                                                                                                                                                |                                                                                                                                                                      |
| Residential                                                                                                                                       |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Individually evaluated for impairment                                                                                        | 194us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         | 6us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember     |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 12,958us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       | 15,071us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 13,152us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                   | 15,077us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                             | 14,157us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 12,343us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |
| Loan balances - Individually evaluated for impairment                                                                                             | 4,022us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                | 2,820us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                          |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Collectively evaluated for impairment                                                                                             | 925,282us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                              | 867,094us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                        |                                                                                                                                                                |                                                                                                                                                                      |
| Loan balances - Loans Acquired with Deteriorated Credit Quality                                                                                   | 4,540us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                             | 1,694us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                       |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | 933,844frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                              | 871,608frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                        |                                                                                                                                                                |                                                                                                                                                                      |
| Finance Leases                                                                                                                                    |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                |                                                                                                                                                                                                                  |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Allowance balances - Collectively evaluated for impairment                                                                                        | 3us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |                                                                                                                                                                                                            |                                                                                                                                                                |                                                                                                                                                                      |
| Total Allowance for Loan Losses                                                                                                                   | 3us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                             |                                                                                                                                                                                                       |                                                                                                                                                           | 3us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |
| Loan balances - Collectively evaluated for impairment                                                                                             | 1,106us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                     | 1,545us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember               |                                                                                                                                                                |                                                                                                                                                                      |
| Loans                                                                                                                                             | $ 1,106frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                                   | $ 1,545frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                             |                                                                                                                                                                |                                                                                                                                                                      |

| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans and Leases Receivable
+ References
No definition available.
+ Details**Name:**frme_LoansAndLeasesReceivable**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were acquired with deteriorated credit quality.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The valuation allowance for financing receivables acquired with deteriorated credit quality.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were collectively evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were individually evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: 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(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section L -Subsection 1
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