| **Fair Value of Financial Instruments - Disclosures About Fair Value of Assets and Liabilities - Additional Information (Detail) (Significant Unobservable Inputs (Level 3), USD $)** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Significant Unobservable Inputs (Level 3)                                                                                                                                             |            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                      |            |
| Trust preferred securities, carrying value                                                                                                                                            | $ 6,100,000     |
| Trust preferred securities, fair value                                                                                                                                                | 194,000         |
| Other-than-temporary impairment ("OTTI") loss                                                                                                                                         | $ 104,000       |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Of Tax Amount, Other Than Temporary Impairment On Debt Security, Catergorized As Available For Sale, Recognized In Other Comprehensive Income (Loss)
+ References
No definition available.
+ Details**Name:**frme_NetOfTaxAmountOtherThanTemporaryImpairmentOnDebtSecurityCatergorizedAsAvailableForSaleRecognizedInOtherComprehensiveIncomeLoss**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trust Preferred Securities Carrying Value
+ References
No definition available.
+ Details**Name:**frme_TrustPreferredSecuritiesCarryingValue**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trust Preferred Securities Fair Value
+ References
No definition available.
+ Details**Name:**frme_TrustPreferredSecuritiesFairValue**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |