| **Commercial Non-Impaired Loans Included in ASC 450 Loss Migration Analysis (Detail) (Nonimpaired Loans, USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                                                           |           |   |
| Number of Loans                                                                                                                                           | 3              | 4 |
| Recorded Balance                                                                                                                                          | $ 282          | $ 934 |
| Commercial and industrial                                                                                                                                 |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                           |           |   |
| Number of Loans                                                                                                                                           | 1              |   |
| Recorded Balance                                                                                                                                          | 5              |   |
| Commercial and farm land                                                                                                                                  |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                           |           |   |
| Number of Loans                                                                                                                                           | 1              | 1 |
| Recorded Balance                                                                                                                                          | 230            | 717 |
| Residential                                                                                                                                               |           |   |
| **Financing Receivable, Impaired [Line Items]**                                                                                                           |           |   |
| Number of Loans                                                                                                                                           | 1              | 3 |
| Recorded Balance                                                                                                                                          | $ 47           | $ 217 |

| 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 |