| **Loans and Allowance - Summary of Troubled Debt Restructurings (Details) $ in Thousands** | 12 Months Ended |
|--------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 5,960         | $ 1,237 |
| Post-Modification Recorded Balance                                                         | $ 6,601         | $ 1,257 |
| Number of Loans | loan                                                                     | 23              | 23 |
| Commercial and industrial loans                                                            |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 4,111         |   |
| Post-Modification Recorded Balance                                                         | $ 2,115         |   |
| Number of Loans | loan                                                                     | 7               |   |
| Construction                                                                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 79            |   |
| Post-Modification Recorded Balance                                                         | $ 80            |   |
| Number of Loans | loan                                                                     | 1               |   |
| Commercial and farmland                                                                    |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 1,281         | $ 259 |
| Post-Modification Recorded Balance                                                         | $ 3,024         | $ 259 |
| Number of Loans | loan                                                                     | 3               | 1 |
| Residential                                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 200           | $ 632 |
| Post-Modification Recorded Balance                                                         | $ 1,113         | $ 622 |
| Number of Loans | loan                                                                     | 10              | 9 |
| Home equity                                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 263           | $ 320 |
| Post-Modification Recorded Balance                                                         | $ 242           | $ 350 |
| Number of Loans | loan                                                                     | 1               | 11 |
| Individuals' loans for household and other personal expenditures                           |            |   |
| **Financing Receivable, Modifications [Line Items]**                                       |            |   |
| Pre-Modification Recorded Balance                                                          | $ 26            | $ 26 |
| Post-Modification Recorded Balance                                                         | $ 27            | $ 26 |
| Number of Loans | loan                                                                     | 1               | 2 |

| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Number of Contracts, During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsNumberOfContractsDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Post-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Pre-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPreModificationRecordedInvestmentDuringPeriod**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:**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=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:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |