| **Loans and Allowance - Subsequent Default (Details) (Nonimpaired Loans, USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |   |
| Number of Loans                                                                                                           | 2              | 3 |
| Recorded Balance                                                                                                          | $ 203          | $ 282 |
| Commercial and industrial loans                                                                                           |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |   |
| Number of Loans                                                                                                           | 1              | 1 |
| Recorded Balance                                                                                                          | 146            | 5 |
| Real estate loans - Commercial and farm land                                                                              |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |   |
| Number of Loans                                                                                                           |           | 1 |
| Recorded Balance                                                                                                          |           | 230 |
| Real estate loans - Residential                                                                                           |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |   |
| Number of Loans                                                                                                           | 1              | 1 |
| Recorded Balance                                                                                                          | $ 57           | $ 47 |

| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Subsequent Default, Number of Contracts During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsSubsequentDefaultNumberOfContractsDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Subsequent Default, Recorded Investment, During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsSubsequentDefaultRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableImpairedLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |