| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | $ 2,908frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod                                                                                                                                                                                                                                           | $ 149frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod |
| Troubled Debt Restructurings, Term Modifications                                                                                            |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,803frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsTermModificationsMember                                                                                                                                |    |
| Troubled Debt Restructurings Rate Modifications                                                                                             |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 26frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateModificationsMember                                                                                                                                   | 134frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateModificationsMember |
| Troubled Debt Restructurings, Combination                                                                                                   |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,079frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateAndTermModificationsMember                                                                                                                         | 15frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateAndTermModificationsMember |
| Commercial and industrial loans                                                                                                             |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,332frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                                                                                                                     |   |
| Commercial and industrial loans | Troubled Debt Restructurings, Term Modifications                                                          |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 253frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsTermModificationsMember          |   |
| Commercial and industrial loans | Troubled Debt Restructurings Rate Modifications                                                           |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                            |   |
| Commercial and industrial loans | Troubled Debt Restructurings, Combination                                                                 |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,079frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateAndTermModificationsMember |   |
| Real estate loans - Construction                                                                                                            |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 2frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                                                                                                                                 |   |
| Real estate loans - Construction | Troubled Debt Restructurings, Term Modifications                                                         |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 2frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsTermModificationsMember                    |   |
| Real estate loans - Commercial and farmland                                                                                                 |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,548frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                                                                                       |   |
| Real estate loans - Commercial and farmland | Troubled Debt Restructurings, Term Modifications                                              |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 1,548frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsTermModificationsMember          |   |
| Real estate loans - Residential                                                                                                             |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 26frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                                                                                            | 134frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Real estate loans - Residential | Troubled Debt Restructurings, Term Modifications                                                          |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                            |    |
| Real estate loans - Residential | Troubled Debt Restructurings Rate Modifications                                                           |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                | 26frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateModificationsMember               | 134frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateModificationsMember |
| Real estate loans - Residential | Troubled Debt Restructurings, Combination                                                                 |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                            |    |
| Individuals' loans for household and other personal expenditures                                                                            |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                                 | 15frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings Rate Modifications                          |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                                 |    |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings, Combination                                |                                                                                                                                                                                                                                                                                                                 |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |                                                                                                                                                                                                                                                                                                                 |   |
| Modification                                                                                                                                |                                                                                                                                                                                                                                                                                                                 | $ 15frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember/ frme_TroubledDebtRestructuringModficationByTypeAxis= frme_TroubledDebtRestructuringsRateAndTermModificationsMember |

| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_TroubledDebtRestructuringModficationByTypeAxis=frme_TroubledDebtRestructuringsRateAndTermModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_TroubledDebtRestructuringModficationByTypeAxis=frme_TroubledDebtRestructuringsRateModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_TroubledDebtRestructuringModficationByTypeAxis=frme_TroubledDebtRestructuringsTermModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |