| **Loans and Allowance - Summary of Non-Accrual Loans by Loan Class (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                              | Dec. 31, 2014                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | $ 44,321us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                      | $ 48,789us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                      |
| Commercial and industrial loans                                                                                                |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 8,249us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember | 7,048us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndIndustrialLoansMember |
| Agricultural production financing and other loans to farmers                                                                   |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 1,978us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember        | 5,800us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_FarmingAndAgricultureMember        |
| Real estate loans - Construction                                                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 1,585us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember         | 1,439us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConstructionLoansMember         |
| Real estate loans - Commercial and farmland                                                                                    |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 16,931us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  | 19,350us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember  |
| Real estate loans - Residential                                                                                                |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 13,314us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | 12,933us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    |
| Real estate loans - Home Equity                                                                                                |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 2,141us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                | 1,988us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_HomeEquityMember                |
| Individuals' loans for household and other personal expenditures                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              | 123us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                | 231us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                |
| Other loans                                                                                                                    |                                                                                                                                                                                       |                                                                                                                                                                                       |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                               |                                                                                                                                                                                       |                                                                                                                                                                                       |
| Non-accrual loans                                                                                                              |                                                                                                                                                                                       |                                                                                                                                                                                  |

| X                                                                                                                                                                            |
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| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                           |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_OtherLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
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| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |