| **Loans and Allowance - Subsequent Default (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 5               | 7 |
| Recorded Balance                                                                                      | $ 1,207         | $ 620 |
| Commercial and industrial loans                                                                       |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 3               | 1 |
| Recorded Balance                                                                                      | 173             | 23 |
| Commercial and farmland                                                                               |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 2               | 2 |
| Recorded Balance                                                                                      | 1,034           | 212 |
| Residential                                                                                           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       |            | 4 |
| Recorded Balance                                                                                      |            | $ 385 |

| 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 |