| **Loans and Allowance - Subsequent Default (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                      |
|-------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                       |                                                                                                                                                                                                 |   |
| Number of Loans                                                                                       | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1                                                                                                                         | 5us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 |
| Recorded Balance                                                                                      | $ 70us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1                                                                                                                     | $ 1,207us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 |
| Commercial and industrial loans                                                                       |                                                                                                                                                                                                 |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |                                                                                                                                                                                                 |   |
| Number of Loans                                                                                       |                                                                                                                                                                                                 | 3us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Recorded Balance                                                                                      |                                                                                                                                                                                                 | 173us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Commercial and farmland                                                                               |                                                                                                                                                                                                 |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |                                                                                                                                                                                                 |   |
| Number of Loans                                                                                       |                                                                                                                                                                                                 | 2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Recorded Balance                                                                                      |                                                                                                                                                                                                 | 1,034us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |
| Residential                                                                                           |                                                                                                                                                                                                 |   |
| **Financing Receivable, Impaired [Line Items]**                                                       |                                                                                                                                                                                                 |   |
| Number of Loans                                                                                       | 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember     |   |
| Recorded Balance                                                                                      | $ 70us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |   |

| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableImpairedLineItems**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_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_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |