| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | $ 523          | $ 4,049        | $ 668 | $ 4,561 |
| Troubled Debt Restructurings, Term Modifications                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 367            | 36             | 367 | 60 |
| Troubled Debt Restructurings Rate Modifications                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 23             | 100            | 154 | 100 |
| Troubled Debt Restructurings, Combination                                                                                                   |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 133            | 3,913          | 147 | 4,401 |
| Commercial and industrial loans                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 36             |   | 126 |
| Commercial and industrial loans | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 36             |   | 60 |
| Commercial and industrial loans | Troubled Debt Restructurings, Combination                                                                 |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           |           |   | 66 |
| Real estate loans - Commercial and farm land                                                                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 272            | 3,549          | 272 | 3,935 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Term Modifications                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 272            |           | 272 |   |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Combination                                                    |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 3,549          |   | 3,935 |
| Real estate loans - Residential                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 217            | 419            | 277 | 455 |
| Real estate loans - Residential | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 95             |           | 95 |   |
| Real estate loans - Residential | Troubled Debt Restructurings Rate Modifications                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 100            | 60 | 100 |
| Real estate loans - Residential | Troubled Debt Restructurings, Combination                                                                 |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 122            | 319            | 122 | 355 |
| Real estate loans - Home equity                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 23             |           | 94 |   |
| Real estate loans - Home equity | Troubled Debt Restructurings Rate Modifications                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 23             |           | 94 |   |
| Individuals' loans for household and other personal expenditures                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 11             | 45             | 25 | 45 |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings Rate Modifications                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           |      |   |   |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings, Combination                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | $ 11           | $ 45           | $ 25 | $ 45 |

| 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 |