| **Loans and Allowance - Summary of Troubled Debt Restructurings by Modification (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | $ 1,766        | $ 2,505        | $ 4,271 | $ 6,695 |
| Troubled Debt Restructurings, Term Modifications                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 1,732          |      | 1,785 | 2,451 |
| Troubled Debt Restructurings Rate Modifications                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 34             | 687            | 94 | 939 |
| Troubled Debt Restructurings, Combination                                                                                                   |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      | 1,818          | 2,392 | 3,305 |
| Commercial and industrial loans                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 175            | 1,048          | 291 | 1,302 |
| Commercial and industrial loans | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 175            |      | 228 | 230 |
| 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                                                                                                                                |      | 1,048          | 63 | 1,072 |
| Real estate loans - Construction                                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 299            |   | 639 |
| 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                                                                                                                                |           | 299            |   | 639 |
| Real estate loans - Commercial and farm land                                                                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 1,399          | 875            | 3,349 | 2,758 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Term Modifications                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 1,399          |      | 1,399 | 1,693 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings Rate Modifications                                              |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      | 680            |    | 680 |
| Real estate loans - Commercial and farm land | Troubled Debt Restructurings, Combination                                                    |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      | 195            | 1,950 | 385 |
| Real estate loans - Residential                                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 192            | 167            | 605 | 1,880 |
| Real estate loans - Residential | Troubled Debt Restructurings, Term Modifications                                                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 158            |      | 158 | 528 |
| Real estate loans - Residential | Troubled Debt Restructurings Rate Modifications                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                | 34             |      | 94 | 252 |
| Real estate loans - Residential | Troubled Debt Restructurings, Combination                                                                 |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |      | 167            | 353 | 1,100 |
| Individuals' loans for household and other personal expenditures                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 116            | 26 | 116 |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings Rate Modifications                          |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | 7              |    | 7 |
| Individuals' loans for household and other personal expenditures | Troubled Debt Restructurings, Combination                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                                        |           |           |   |   |
| Modification                                                                                                                                |           | $ 109          | $ 26 | $ 109 |

| 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 |