| **Core Deposit and Other Intangibles - Narrative (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|---------------------------------------------------------------------------------------|----------------|
||
| **Finite-Lived Intangible Assets [Line Items]**                                       |           |   |
| Measurement period adjustment                                                         | $ 2,142        |  |
| Ameriana                                                                              |           |   |
| **Finite-Lived Intangible Assets [Line Items]**                                       |           |   |
| Measurement period adjustment                                                         | $ 2,142        |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FiniteLivedIntangibleAssetsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) to assets, excluding financial assets and goodwill, lacking physical substance with a finite life for purchase accounting adjustments.
+ References
No definition available.
+ Details**Name:**us-gaap_FiniteLivedIntangibleAssetsPurchaseAccountingAdjustments**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=frme_AmerianaBancorpMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |