| **Investment Securities - Gross Gains on Sales and Redemptions of Available for Sale Securities (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Sales and Redemptions of Available for Sale Securities:**                                                                          |           |   |
| Gross gains                                                                                                                          | $ 1,609        | $ 598 |
| Gross losses                                                                                                                         |                |   |

| X                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Securities, Available-for-sale, Realized Gain
+ References
No definition available.
+ Details**Name:**frme_DebtSecuritiesAvailableforsaleRealizedGain**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Securities, Available-for-sale, Realized Loss
+ References
No definition available.
+ Details**Name:**frme_DebtSecuritiesAvailableforsaleRealizedLoss**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |