| **Loans and Allowance - Summary of Troubled Debt Restructurings (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           | $ 4,986        | $ 3,120        | $ 5,629 | $ 5,361 |
| Post-Modification Recorded Balance                                                                                          | 4,051          | 2,849          | 4,616 | 4,946 |
| Number of Loans                                                                                                             | 10             | 18             | 16 | 26 |
| Commercial and industrial loans                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           | 36             | 166            | 133 | 405 |
| Post-Modification Recorded Balance                                                                                          | 36             | 166            | 133 | 405 |
| Number of Loans                                                                                                             | 1              | 2              | 4 | 4 |
| Real estate loans - Construction                                                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           |           | 491            |   | 491 |
| Post-Modification Recorded Balance                                                                                          |           | 350            |   | 350 |
| Number of Loans                                                                                                             |           | 1              |   | 1 |
| Real estate loans - Commercial and farm land                                                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           | 4,474          | 730            | 4,985 | 2,508 |
| Post-Modification Recorded Balance                                                                                          | 3,550          | 735            | 3,981 | 2,369 |
| Number of Loans                                                                                                             | 2              | 4              | 4 | 6 |
| Real estate loans - Residential                                                                                             |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           | 432            | 1,733          | 467 | 1,957 |
| Post-Modification Recorded Balance                                                                                          | 420            | 1,598          | 457 | 1,822 |
| Number of Loans                                                                                                             | 5              | 11             | 6 | 15 |
| Individuals' loans for household and other personal expenditures                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |           |           |   |   |
| Pre-Modification Recorded Balance                                                                                           | 44             |           | 44 |   |
| Post-Modification Recorded Balance                                                                                          | $ 45           |           | $ 45 |   |
| Number of Loans                                                                                                             | 2              |           | 2 |   |

| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Number of Contracts, During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsNumberOfContractsDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, ModificationsPost-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Pre-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPreModificationRecordedInvestmentDuringPeriod**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 |