| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification Type (Details) - USD ($) $ in Thousands** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | $ 4,788        | $ 1,603        | $ 5,494 | $ 5,042 |
| Term Modification                                                                                                         |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 1,559          | 1,342          | 1,559 | 3,725 |
| Rate Modification                                                                                                         |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 143            | 21             | 304 | 47 |
| Combination                                                                                                               |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 3,086          | 240            | 3,631 | 1,270 |
| Commercial and industrial loans                                                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           | 492            | 198 | 2,264 |
| Commercial and industrial loans | Term Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           | 492            |  | 1,234 |
| Commercial and industrial loans | Rate Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           |                |  |  |
| Commercial and industrial loans | Combination                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           |                | 198 | 1,030 |
| Agricultural production financing and other loans to farmers                                                              |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 1,141          |           | 1,190 |   |
| Agricultural production financing and other loans to farmers | Term Modification                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 1,141          |           | 1,141 |   |
| Agricultural production financing and other loans to farmers | Rate Modification                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |                |           | 49 |   |
| Agricultural production financing and other loans to farmers | Combination                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |                |           |  |   |
| Real estate loans - Construction                                                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           |           |   | 199 |
| Real estate loans - Construction | Term Modification                                                                      |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           |           |   | 199 |
| 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                                                                                                              | 3,504          | 240            | 3,851 | 1,682 |
| Real estate loans - Commercial and farmland | Term Modification                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 418            |                | 418 | 1,442 |
| 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                                                                                                              | 3,086          | 240            | 3,433 | 240 |
| Real estate loans - Residential                                                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           | 871            | 112 | 897 |
| Real estate loans - Residential | Term Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           | 850            |  | 850 |
| Real estate loans - Residential | Rate Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           | 21             | 112 | 47 |
| Real estate loans - Residential | Combination                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |           |                |  |  |
| Real estate loans - Home Equity                                                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 143            |           | 143 |   |
| Real estate loans - Home Equity | Term Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              |                |           |  |   |
| Real estate loans - Home Equity | Rate Modification                                                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                      |           |           |   |   |
| Modification                                                                                                              | 143            |           | 143 |   |
| Real estate loans - Home Equity | 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                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_TroubledDebtRestructuringModficationByTypeAxis=frme_TroubledDebtRestructuringsRateModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| 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_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |