| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification Type (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 1,078        | $ 2,908 |
| Term Modification                                                                                                         |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                | 1,803 |
| Rate Modification                                                                                                         |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 467          | 26 |
| Combination                                                                                                               |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | 611            | 1,079 |
| Commercial and industrial loans                                                                                           |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 260          | 1,332 |
| Commercial and industrial loans | Term Modification                                                                       |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                | 253 |
| Commercial and industrial loans | Rate Modification                                                                       |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                |   |
| Commercial and industrial loans | Combination                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 260          | 1,079 |
| Agricultural production financing and other loans to farmers                                                              |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | 331            |   |
| Agricultural production financing and other loans to farmers | Rate Modification                                          |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | 331            |   |
| Real estate loans - Construction                                                                                          |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |           | 2 |
| Real estate loans - Construction | Term Modification                                                                      |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |           | $ 2 |
| Real estate loans - Construction | Rate Modification                                                                      |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |           |  |
| Real estate loans - Construction | Combination                                                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |           |  |
| Real estate loans - Commercial and farmland                                                                               |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 351          | $ 1,548 |
| Real estate loans - Commercial and farmland | Term Modification                                                           |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                | $ 1,548 |
| Real estate loans - Commercial and farmland | Rate Modification                                                           |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                |  |
| Real estate loans - Commercial and farmland | Combination                                                                 |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 351          |  |
| Real estate loans - Residential                                                                                           |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 123          | $ 26 |
| Real estate loans - Residential | Term Modification                                                                       |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                |  |
| Real estate loans - Residential | Rate Modification                                                                       |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 123          | $ 26 |
| Real estate loans - Residential | Combination                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                |  |
| Individuals' loans for household and other personal expenditures                                                          |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 13           |   |
| Individuals' loans for household and other personal expenditures | Rate Modification                                      |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              | $ 13           |   |
| Individuals' loans for household and other personal expenditures | Combination                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |   |
| Modification                                                                                                              |                |   |

| 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                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ 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_TroubledDebtRestructuringsTermModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_TroubledDebtRestructuringModficationByTypeAxis=frme_TroubledDebtRestructuringsRateAndTermModificationsMember**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=frme_FarmingAndAgricultureMember**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=frme_CommercialAndFarmLandLoansMember**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:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |