| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification Type (Details) $ in Thousands** | 3 Months Ended |
|-----------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | $ 90           |
| Rate Modification                                                                                               |           |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | 90             |
| Combination                                                                                                     |           |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | 0              |
| Residential                                                                                                     |           |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | 90             |
| Residential | Rate Modification                                                                                 |           |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | 90             |
| Residential | Combination                                                                                       |           |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**                                              |           |
| Modification                                                                                                    | $ 0            |

| 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_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=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |