| **Loans and Allowance - Subsequent Default (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 7               | 14 |
| Recorded Balance                                                                                      | $ 620           | $ 2,984 |
| Commercial and industrial                                                                             |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 1               | 3 |
| Recorded Balance                                                                                      | 23              | 471 |
| Commercial and farm land                                                                              |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 2               | 3 |
| Recorded Balance                                                                                      | 212             | 1,951 |
| Residential                                                                                           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       | 4               | 7 |
| Recorded Balance                                                                                      | 385             | 557 |
| Consumer                                                                                              |            |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |            |   |
| Number of Loans                                                                                       |            | 1 |
| Recorded Balance                                                                                      |            | $ 5 |

| 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 |