| **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                                                                                                                                | $ 149          | $ 552 |
| Troubled Debt Restructurings, Term Modifications                                                                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 31 |
| Troubled Debt Restructurings Rate Modifications                                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | 134            |   |
| Troubled Debt Restructurings, Combination                                                                                                   |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | 15             | 521 |
| Commercial and industrial loans                                                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 100 |
| Commercial and industrial loans | Troubled Debt Restructurings, Term Modifications                                                          |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 31 |
| Commercial and industrial loans | Troubled Debt Restructurings, Combination                                                                 |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 69 |
| Real estate loans - Commercial and farm land                                                                                                |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 415 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Combination                                                    |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 415 |
| Real estate loans - Residential                                                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | 134            | 37 |
| Real estate loans - Residential | Troubled Debt Restructurings Rate Modifications                                                           |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | 134            |   |
| Real estate loans - Residential | Troubled Debt Restructurings, Combination                                                                 |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                |           | 37 |
| Individuals' loans for household and other personal expenditures                                                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | 15             |   |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings, Combination                                |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |   |
| Modification                                                                                                                                | $ 15           |   |

| 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 |