| **Loans and Allowance - Summary of Troubled Debt Restructurings (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Pre-Modification Recorded Balance                                                                                           | $ 2,465us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2                                                                                                                                | $ 146us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 |
| Post-Modification Recorded Balance                                                                                          | 3,208us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2                                                                                                                                 | 149us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 |
| Number of Loans                                                                                                             | 7us-gaap_FinancingReceivableModificationsNumberOfContracts2                                                                                                                                                      | 4us-gaap_FinancingReceivableModificationsNumberOfContracts2 |
| Commercial and industrial loans                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Financing Receivable Modifications Pre Modification Recorded Investment During Period                                       | 2,362frme_FinancingReceivableModificationsPreModificationRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember  |   |
| Financing Receivable Modifications Post Modification Recorded Investment During Period                                      | 1,361frme_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |   |
| Financing Receivable Modifications Number Of Contracts During Period                                                        | 4frme_FinancingReceivableModificationsNumberOfContractsDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember                      |   |
| Real estate loans - Construction                                                                                            |                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Financing Receivable Modifications Pre Modification Recorded Investment During Period                                       | 79frme_FinancingReceivableModificationsPreModificationRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember             |   |
| Financing Receivable Modifications Post Modification Recorded Investment During Period                                      | 80frme_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember            |   |
| Financing Receivable Modifications Number Of Contracts During Period                                                        | 1frme_FinancingReceivableModificationsNumberOfContractsDuringPeriod/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember                              |   |
| Real estate loans - Commercial and farmland                                                                                 |                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Pre-Modification Recorded Balance                                                                                           |                                                                                                                                                                                                        |   |
| Post-Modification Recorded Balance                                                                                          | 1,743us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember           |   |
| Number of Loans                                                                                                             | 1us-gaap_FinancingReceivableModificationsNumberOfContracts2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                |   |
| Real estate loans - Residential                                                                                             |                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Pre-Modification Recorded Balance                                                                                           | 24us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                 | 131us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Post-Modification Recorded Balance                                                                                          | 24us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                | 134us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Number of Loans                                                                                                             | 1us-gaap_FinancingReceivableModificationsNumberOfContracts2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                  | 3us-gaap_FinancingReceivableModificationsNumberOfContracts2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Individuals' loans for household and other personal expenditures                                                            |                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                                        |                                                                                                                                                                                                             |   |
| Pre-Modification Recorded Balance                                                                                           |                                                                                                                                                                                                             | 15us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Post-Modification Recorded Balance                                                                                          |                                                                                                                                                                                                             | $ 15us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Number of Loans                                                                                                             |                                                                                                                                                                                                             | 1us-gaap_FinancingReceivableModificationsNumberOfContracts2/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |

| 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, Modifications, Post-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                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsNumberOfContracts2**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Post-modification amount of investment in financing receivables modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |