| **Loans and Allowance - Subsequent Default (Details) $ in Thousands** | 3 Months Ended | 6 Months Ended |
|-----------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**    |           |           |   |   |
| Number of Loans | loan                                                | 1              | 1              | 1 | 1 |
| Recorded Balance | $                                                  | $ 268          | $ 62           | $ 268 | $ 62 |
| Commercial and industrial loans                                       |           |           |   |   |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**    |           |           |   |   |
| Number of Loans | loan                                                | 1              |           | 1 |   |
| Recorded Balance | $                                                  | $ 268          |           | $ 268 |   |
| Residential                                                           |           |           |   |   |
| **Financing Receivable, Troubled Debt Restructuring [Line Items]**    |           |           |   |   |
| Number of Loans | loan                                                |           | 1              |   | 1 |
| Recorded Balance | $                                                  |           | $ 62           |   | $ 62 |

| 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 receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=121611835&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 financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=121611835&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:** |