| **Loans and Allowance - Narrative (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                 | 12 Months Ended                                  |              |
|----------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------|--------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Loans held for sale                                                                          | $ 7,235us-gaap_LoansReceivableHeldForSaleNet                                                                                                                                   |                                             |         |   | $ 5,331us-gaap_LoansReceivableHeldForSaleNet |   |   |   | $ 7,235us-gaap_LoansReceivableHeldForSaleNet | $ 5,331us-gaap_LoansReceivableHeldForSaleNet |   |   |
| Allowance for loan losses                                                                    | 63,964us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                |                                             |         |   | 67,870us-gaap_LoansAndLeasesReceivableAllowance |   |   |   | 63,964us-gaap_LoansAndLeasesReceivableAllowance | 67,870us-gaap_LoansAndLeasesReceivableAllowance | 69,366us-gaap_LoansAndLeasesReceivableAllowance | 70,898us-gaap_LoansAndLeasesReceivableAllowance |
| Decrease in allowance for loan losses                                                        |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 3,906us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease |   |   |   |
| Increase in reserve on impaired loans                                                        |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 1,186frme_ImpairedFinancingReceivableRelatedAllowancePeriodIncreaseDecrease |   |   |   |
| Reserve on impaired loans                                                                    | 2,769us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                       |                                             |         |   | 1,583us-gaap_ImpairedFinancingReceivableRelatedAllowance |   |   |   | 2,769us-gaap_ImpairedFinancingReceivableRelatedAllowance | 1,583us-gaap_ImpairedFinancingReceivableRelatedAllowance | 4,243us-gaap_ImpairedFinancingReceivableRelatedAllowance |   |
| Net charge offs                                                                              |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 6,466us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet |   |   |   |
| Decrease in net charge offs                                                                  |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 1,678frme_AllowanceForLoanAndLeaseLossesWriteOffsNetPeriodIncreaseDecrease |   |   |   |
| Provision for loan losses                                                                    | 960us-gaap_ProvisionForLoanLeaseAndOtherLosses                                                                                                                                 | 1,600us-gaap_ProvisionForLoanLeaseAndOtherLosses |    |    | 1,016us-gaap_ProvisionForLoanLeaseAndOtherLosses | 1,533us-gaap_ProvisionForLoanLeaseAndOtherLosses | 1,997us-gaap_ProvisionForLoanLeaseAndOtherLosses | 2,102us-gaap_ProvisionForLoanLeaseAndOtherLosses | 2,560us-gaap_ProvisionForLoanLeaseAndOtherLosses | 6,648us-gaap_ProvisionForLoanLeaseAndOtherLosses | 18,534us-gaap_ProvisionForLoanLeaseAndOtherLosses |   |
| Decrease in provision for loan losses                                                        |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 4,088frme_ProvisionforLoanLeaseandOtherLossesPeriodIncreaseDecrease |   |   |   |
| Commercial impaired loan total                                                               | 116,223us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                   |                                             |         |   | 119,755us-gaap_ImpairedFinancingReceivableRecordedInvestment |   |   |   | 116,223us-gaap_ImpairedFinancingReceivableRecordedInvestment | 119,755us-gaap_ImpairedFinancingReceivableRecordedInvestment | 79,179us-gaap_ImpairedFinancingReceivableRecordedInvestment |   |
| Merger with Community Bancshares, Inc.                                                       |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Commercial impaired loan total                                                               | 17,027us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                     |                                             |         |   |   |   |   |   | 17,027us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |   |   |
| Merger with CFS Bancorp, Inc. (Citizens)                                                     |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Commercial impaired loan total                                                               |                                                                                                                                                                           |                                             |         |   | 69,448us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |   |   |   |   | 69,448us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |   |   |
| SCB Bank                                                                                     |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Commercial impaired loan total                                                               |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   | 17,334us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |   |
| Commercial and farmland                                                                      |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Reserve on impaired loans                                                                    | 1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |                                             |         |   | 6us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |   |   |   | 1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 6us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember | 2,460us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |   |
| Percentage of troubled debt restructured loans                                               |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 49.00%frme_FinancingReceivableModificationsPostModificationLoansAsPercentageOfTroubledDebtRestructuredLoans/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |   |   |   |
| Residential                                                                                  |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                             |                                                                                                                                                                           |                                             |         |   |   |   |   |   |   |   |   |   |
| Allowance for loan losses                                                                    | 13,152us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember            |                                             |         |   | 15,077us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   |   |   | 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 |
| Reserve on impaired loans                                                                    | 194us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember     |                                             |         |   |   |   |   |   | 194us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   | 50us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   |
| Provision for loan losses                                                                    |                                                                                                                                                                           |                                             |         |   |   |   |   |   | $ (839)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | $ 3,514us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | $ 6,176us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   |
| Percentage of troubled debt restructured loans                                               |                                                                                                                                                                           |                                             |         |   |   |   |   |   | 28.00%frme_FinancingReceivableModificationsPostModificationLoansAsPercentageOfTroubledDebtRestructuredLoans/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Allowance For Loan And Lease Losses Write Offs, Net, Period Increase (Decrease)
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| - Definition
Financing Receivable, Modifications, Post-Modification, Loans as Percentage of Troubled Debt Restructured Loans
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| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired Financing Receivable Related Allowance, Period Increase (Decrease)
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| X                                                                                                                                                                                                                                                                                                                       |
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| - Definition
Provision for Loan, Lease, and Other Losses, Period Increase (Decrease)
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) of allowance reserve for credit loss.
+ References
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| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after recoveries, of loans and leases that have been written-off.
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No definition available.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
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| - Definition
Amount of allowance for credit losses related to recorded investment.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.
+ References
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