| **Loans and Allowance - Subsequent Default (Details) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Modifications [Line Items]**                  |           |           |   |   |
| Number of Loans | loan                                                | 1              | 1              | 4 | 1 |
| Recorded Balance | $                                                  | $ 54           | $ 21           | $ 251 | $ 21 |
| Commercial and industrial loans                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                  |           |           |   |   |
| Number of Loans | loan                                                |                |           | 3 |   |
| Recorded Balance | $                                                  |                |           | $ 197 |   |
| Real estate loans - Residential                                       |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                  |           |           |   |   |
| Number of Loans | loan                                                | 1              |           | 1 |   |
| Recorded Balance | $                                                  | $ 54           |           | $ 54 |   |
| Nonimpaired loans | Real estate loans - Residential                   |           |           |   |   |
| **Financing Receivable, Modifications [Line Items]**                  |           |           |   |   |
| Number of Loans | loan                                                |           | 1              |   | 1 |
| Recorded Balance | $                                                  |           | $ 21           |   | $ 21 |

| 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 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=66008989&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=66008989&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_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:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InternalCreditAssessmentAxis=frme_NonimpairedLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |