| **Loans and Allowance - Subsequent Default (Details) (Nonimpaired Loans, USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           | 1              | 7              | 2 | 9 |
| Recorded Balance                                                                                                          | $ 3            | $ 2,728        | $ 226 | $ 3,532 |
| Commercial and industrial loans                                                                                           |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           | 1              |      | 1 | 1 |
| Recorded Balance                                                                                                          | 3              |      | 3 | 46 |
| Real estate loans - Commercial and farm land                                                                              |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           |      | 2              | 1 | 3 |
| Recorded Balance                                                                                                          |      | 445            | 223 | 1,203 |
| Real estate loans - Residential                                                                                           |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           |           | 5              |   | 5 |
| Recorded Balance                                                                                                          |           | $ 2,283        |   | $ 2,283 |

| 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 |