| **Loans and Allowance - Subsequent Default (Details) (Nonimpaired Loans, USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           |           | 11             |   | 15 |
| Recorded Balance                                                                                                          |           | $ 1,809        |   | $ 3,932 |
| Commercial and industrial loans                                                                                           |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           | 1              | 3              | 1 | 3 |
| Recorded Balance                                                                                                          | 18             | 1,415          | 18 | 1,415 |
| Real estate loans - Commercial and farm land                                                                              |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           |      | 1              | 1 | 1 |
| Recorded Balance                                                                                                          |      | 23             | 223 | 23 |
| Real estate loans - Residential                                                                                           |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           | 2              | 7              | 2 | 11 |
| Recorded Balance                                                                                                          | 166            | 371            | 166 | 2,494 |
| Real estate loans - Home equity                                                                                           |           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                           |           |           |   |   |
| Number of Loans                                                                                                           | 3              |           | 4 |   |
| Recorded Balance                                                                                                          | $ 184          |           | $ 407 |   |

| 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 |