| **Loans and Allowance - Summary of Non-Accrual Loans by Loan Class (Details) - USD ($) $ in Thousands** | Mar. 31, 2016 | Dec. 31, 2015 |
|---------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | $ 36,719      | $ 31,389      |
| Commercial and industrial loans                                                                         |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 3,694         | 4,634         |
| Agricultural production financing and other loans to farmers                                            |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 1,535         | 827           |
| Real estate loans - Construction                                                                        |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 731           | 736           |
| Real estate loans - Commercial and farmland                                                             |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 16,440        | 11,277        |
| Real estate loans - Residential                                                                         |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 11,674        | 11,818        |
| Real estate loans - Home Equity                                                                         |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | 2,449         | 1,952         |
| Individuals' loans for household and other personal expenditures                                        |          |          |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                        |          |          |
| Non-accrual loans                                                                                       | $ 196         | $ 145         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ 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=49124121&loc=d3e5111-111524
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| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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