| **Nature of Operations and Summary of Significant Accounting Policies - Allowance for Loan Losses (Details)** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Accounting Policies [Abstract]**                                                                            |            |
| Number of key element used in assessing the appropriateness of the allowance for loan losses | key_element    | 3               |
| Commercial relationships threshold that exhibit credit weaknesses | $                                         | $ 500,000       |

| X                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Allowance for Loan Losses, Number Of Key Elements Applied to Determination Of Valuation Allowance
+ References
No definition available.
+ Details**Name:**frme_AllowanceForLoanLossesNumberOfKeyElementsAppliedToDeterminationOfValuationAllowance**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commercial Banking Relationship, Credit Quality Weakness, Threshold for Individual Review
+ References
No definition available.
+ Details**Name:**frme_CommercialBankingRelationshipCreditQualityWeaknessThresholdforIndividualReview**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_AccountingPoliciesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |