| **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                                                                                                                                | $ 4,049        | $ 2,567        | $ 4,561 | $ 4,664 |
| Troubled Debt Restructurings, Term Modifications                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 36             | 613            | 60 | 2,486 |
| Troubled Debt Restructurings Rate Modifications                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 100            | 258            | 100 | 258 |
| Troubled Debt Restructurings, Combination                                                                                                   |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 3,913          | 1,696          | 4,401 | 1,920 |
| Commercial and industrial loans                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 36             | 31             | 126 | 269 |
| Commercial and industrial loans | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 36             |      | 60 | 238 |
| Commercial and industrial loans | Troubled Debt Restructurings Rate Modifications                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      |      |    |    |
| Commercial and industrial loans | Troubled Debt Restructurings, Combination                                                                 |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      | 31             | 66 | 31 |
| Real estate loans - Construction                                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 346            |   | 346 |
| Real estate loans - Construction | Troubled Debt Restructurings Rate Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           |      |   |    |
| Real estate loans - Construction | Troubled Debt Restructurings, Combination                                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 346            |   | 346 |
| Real estate loans - Commercial and farm land                                                                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 3,549          | 681            | 3,935 | 2,316 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Term Modifications                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 82             |    | 1,717 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings Rate Modifications                                              |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      |      |    |    |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Combination                                                    |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 3,549          | 599            | 3,935 | 599 |
| Real estate loans - Residential                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 419            | 1,509          | 455 | 1,733 |
| Real estate loans - Residential | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 531            |    | 531 |
| Real estate loans - Residential | Troubled Debt Restructurings Rate Modifications                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 100            | 258            | 100 | 258 |
| Real estate loans - Residential | Troubled Debt Restructurings, Combination                                                                 |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 319            | 720            | 355 | 944 |
| Individuals' loans for household and other personal expenditures                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 45             |           | 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                                                                                                                                | $ 45           |           | $ 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 |