| **Loans and Allowance - Subsequent Default (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                                                   |
|--------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                   |                                                                                                                                                                                                                                                             |   |
| Number of Loans                                                                                        | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1                                                                                                                                                                                     | 2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 |
| Recorded Balance                                                                                       | $ 6us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1                                                                                                                                                                                  | $ 203us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 |
| Commercial and industrial loans                                                                        |                                                                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                   |                                                                                                                                                                                                                                                             |   |
| Number of Loans                                                                                        |                                                                                                                                                                                                                                                             | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis= frme_CommercialAndIndustrialLoansMember |
| Recorded Balance                                                                                       |                                                                                                                                                                                                                                                             | 146us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis= frme_CommercialAndIndustrialLoansMember |
| Nonimpaired Loans | Real estate loans - Home Equity                                                    |                                                                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                   |                                                                                                                                                                                                                                                             |   |
| Number of Loans                                                                                        | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_HomeEquityRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_NonimpairedLoansMember  |   |
| Recorded Balance                                                                                       | 6us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_HomeEquityRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_NonimpairedLoansMember |   |
| Nonimpaired Loans | Real estate loans - Residential                                                    |                                                                                                                                                                                                                                                             |   |
| **Financing Receivable, Modifications [Line Items]**                                                   |                                                                                                                                                                                                                                                             |   |
| Number of Loans                                                                                        |                                                                                                                                                                                                                                                             | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_NonimpairedLoansMember |
| Recorded Balance                                                                                       |                                                                                                                                                                                                                                                             | $ 57us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember/ us-gaap_InternalCreditAssessmentAxis= frme_NonimpairedLoansMember |

| 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 within the previous 12 months and for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableTroubledDebtRestructuringAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_NonimpairedLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |