| **Summary of Troubled Debt Restructurings (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                 |           |   |
| Pre-Modification Recorded Balance                                                                    | $ 644          | $ 2,236 |
| Post-Modification Recorded Balance                                                                   | 565            | 2,097 |
| Number of Loans                                                                                      | 6              | 8 |
| Commercial and industrial                                                                            |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                 |           |   |
| Pre-Modification Recorded Balance                                                                    | 96             | 238 |
| Post-Modification Recorded Balance                                                                   | 96             | 238 |
| Number of Loans                                                                                      | 3              | 2 |
| Commercial and farm land                                                                             |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                 |           |   |
| Pre-Modification Recorded Balance                                                                    | 511            | 1,774 |
| Post-Modification Recorded Balance                                                                   | 431            | 1,635 |
| Number of Loans                                                                                      | 2              | 2 |
| Residential                                                                                          |           |   |
| **Financing Receivable, Modifications [Line Items]**                                                 |           |   |
| Pre-Modification Recorded Balance                                                                    | 37             | 224 |
| Post-Modification Recorded Balance                                                                   | $ 38           | $ 224 |
| Number of Loans                                                                                      | 1              | 4 |

| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Number of Contracts, During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsNumberOfContractsDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, ModificationsPost-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Pre-Modification Recorded Investment During Period
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPreModificationRecordedInvestmentDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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