| **Loans and Allowance - Allowance for Credit Losses and Loan Portfolio by Loan Segment (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                         | Dec. 31, 2014                                                                                                                                                                                                         | Mar. 31, 2014                                                                                                                                                       | Dec. 31, 2013                                                                                                                                                       |
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| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Individually evaluated for impairment                                                                                         | $ 4,076us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1                                                                                                                         | $ 2,119us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1                                                                                                                         |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 58,245us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment                                                                                                                           | 61,195us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment                                                                                                                           |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Loans acquired with deteriorated credit quality                                                                               | 480us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality                                                                                                                           | 650us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality                                                                                                                           |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 62,801us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                       | 63,964us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                       | 69,583us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     | 67,870us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     |
| Loan balances - Individually evaluated for impairment                                                                                              | 37,398us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment                                                                                                                                                   | 44,093us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment                                                                                                                                                   |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Collectively evaluated for impairment                                                                                              | 3,860,289us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment                                                                                                                                                | 3,807,707us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment                                                                                                                                                |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Loans acquired with deteriorated credit quality                                                                                    | 67,845us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality                                                                                                                                                | 73,065us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality                                                                                                                                                |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | 3,965,532frme_LoansAndLeasesReceivable                                                                                                                                                                                | 3,924,865frme_LoansAndLeasesReceivable                                                                                                                                                                                |                                                                                                                                                                |                                                                                                                                                                |
| Commercial                                                                                                                                         |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Individually evaluated for impairment                                                                                         | 2,700us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              | 1,455us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 27,307us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              | 27,369us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember              |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Loans acquired with deteriorated credit quality                                                                               |                                                                                                                                                                                                             |                                                                                                                                                                                                             |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 30,007us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                          | 28,824us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                          | 30,907us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember        | 27,176us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember        |
| Loan balances - Individually evaluated for impairment                                                                                              | 11,432us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                      | 16,108us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                      |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Collectively evaluated for impairment                                                                                              | 1,063,822us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                   | 1,011,122us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                   |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Loans acquired with deteriorated credit quality                                                                                    | 8,383us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                    | 9,403us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                    |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | 1,083,637frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                                   | 1,036,633frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                                                                   |                                                                                                                                                                |                                                                                                                                                                |
| Commercial Real Estate                                                                                                                             |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Individually evaluated for impairment                                                                                         | 1,017us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        | 470us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember          |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 15,081us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        | 18,207us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Loans acquired with deteriorated credit quality                                                                               | 285us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        | 650us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember        |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 16,383us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                    | 19,327us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                    | 22,358us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember  | 23,102us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember  |
| Loan balances - Individually evaluated for impairment                                                                                              | 21,668us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                | 23,963us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Collectively evaluated for impairment                                                                                              | 1,806,864us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             | 1,796,797us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Loans acquired with deteriorated credit quality                                                                                    | 54,922us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             | 59,122us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                             |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | 1,883,454frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                             | 1,879,882frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember                                                             |                                                                                                                                                                |                                                                                                                                                                |
| Consumer                                                                                                                                           |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 3,138us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 | 2,658us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                 |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 3,138us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                             | 2,658us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                             | 2,410us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           | 2,515us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember           |
| Loan balances - Collectively evaluated for impairment                                                                                              | 70,223us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                        | 73,400us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                        |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | 70,223frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                        | 73,400frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                                                                        |                                                                                                                                                                |                                                                                                                                                                |
| Residential                                                                                                                                        |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Individually evaluated for impairment                                                                                         | 359us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         | 194us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 12,715us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       | 12,958us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Loans acquired with deteriorated credit quality                                                                               | 195us-gaap_FinancingReceivableAllowanceForCreditLossesAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 13,269us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                   | 13,152us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                   | 13,908us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 15,077us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Loan balances - Individually evaluated for impairment                                                                                              | 4,298us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                | 4,022us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                |                                                                                                                                                                |                                                                                                                                                                |
| Loan balances - Collectively evaluated for impairment                                                                                              | 918,527us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                              | 925,282us-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                             | 4,540us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                             |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | 927,365frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                              | 933,844frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                              |                                                                                                                                                                |                                                                                                                                                                |
| Finance Leases                                                                                                                                     |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| **Financing Receivable, Allowance for Credit Losses [Line Items]**                                                                                 |                                                                                                                                                                                                                  |                                                                                                                                                                                                                  |                                                                                                                                                                |                                                                                                                                                                |
| Allowance balances - Collectively evaluated for impairment                                                                                         | 4us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember | 3us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |                                                                                                                                                                |                                                                                                                                                                |
| Total Allowance for Loan Losses                                                                                                                    | 4us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                             | 3us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                             |                                                                                                                                                           |                                                                                                                                                           |
| Loan balances - Collectively evaluated for impairment                                                                                              | 853us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                       | 1,106us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                     |                                                                                                                                                                |                                                                                                                                                                |
| Loans                                                                                                                                              | $ 853frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                                     | $ 1,106frme_LoansAndLeasesReceivable/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember                                                   |                                                                                                                                                                |                                                                                                                                                                |

| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans and Leases Receivable
+ References
No definition available.
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| 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
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| 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
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| 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were individually evaluated for impairment.
+ References
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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
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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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_FinanceLeasesFinancingReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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