| **Loans and Allowance - Summary of Troubled Debt Restructurings (Details) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          | $ 753          | $ 1,357        | $ 6,797 | $ 5,765 |
| Post-Modification Recorded Balance                                                         | $ 831          | $ 1,217        | $ 6,702 | $ 6,370 |
| Number of Loans | loan                                                                     | 14             | 5              | 32 | 16 |
| Commercial and industrial loans                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          |                | $ 363          | $ 260 | $ 4,111 |
| Post-Modification Recorded Balance                                                         |                | $ 218          | $ 260 | $ 2,115 |
| Number of Loans | loan                                                                     |                | 2              | 3 | 7 |
| Agricultural production financing and other loans to farmers                               |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          |                |           | $ 1,606 |   |
| Post-Modification Recorded Balance                                                         |                |           | $ 1,472 |   |
| Number of Loans | loan                                                                     |                |           | 5 |   |
| Real estate loans - Construction                                                           |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          |           |                |   | $ 79 |
| Post-Modification Recorded Balance                                                         |           |                |   | $ 80 |
| Number of Loans | loan                                                                     |           |                |   | 1 |
| Real estate loans - Commercial and farmland                                                |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          | $ 87           | $ 744          | $ 3,978 | $ 1,281 |
| Post-Modification Recorded Balance                                                         | $ 95           | $ 744          | $ 3,955 | $ 3,025 |
| Number of Loans | loan                                                                     | 1              | 1              | 7 | 3 |
| Real estate loans - Residential                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          | $ 611          | $ 11           | $ 724 | $ 55 |
| Post-Modification Recorded Balance                                                         | $ 678          | $ 13           | $ 811 | $ 908 |
| Number of Loans | loan                                                                     | 11             | 1              | 14 | 4 |
| Real estate loans - Home Equity                                                            |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                                       |           |           |   |   |
| Pre-Modification Recorded Balance                                                          | $ 55           | $ 239          | $ 229 | $ 239 |
| Post-Modification Recorded Balance                                                         | $ 58           | $ 242          | $ 204 | $ 242 |
| Number of Loans | loan                                                                     | 2              | 1              | 3 | 1 |

| 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_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=66008989&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=66008989&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=66008989&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_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**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=frme_CommercialAndFarmLandLoansMember**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:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |