| **LOANS AND ALLOWANCE - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands** | Jun. 30, 2023 | Dec. 31, 2022 |
|-------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | $ 12,270,233  | $ 12,003,894  |
| Loans > 90 Days or More Past Due And Accruing                                                            | 428           | 1,737         |
| Current                                                                                                     |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 12,199,643    | 11,952,887    |
| 30-59 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 35,860        | 29,051        |
| 60-89 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 5,660         | 5,935         |
| 90 Days or More Past Due                                                                                    |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 29,070        | 16,021        |
| Commercial and industrial loans                                                                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 3,531,395     | 3,437,126     |
| Loans > 90 Days or More Past Due And Accruing                                                            | 278           | 1,147         |
| Commercial and industrial loans | Current                                                                   |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 3,515,917     | 3,429,314     |
| Commercial and industrial loans | 30-59 Days Past Due                                                       |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 5,165         | 4,904         |
| Commercial and industrial loans | 60-89 Days Past Due                                                       |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 492           | 434           |
| Commercial and industrial loans | 90 Days or More Past Due                                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 9,821         | 2,474         |
| Agricultural land, production and other loans to farmers                                                    |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 230,003       | 241,793       |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Agricultural land, production and other loans to farmers | Current                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 229,976       | 241,739       |
| Agricultural land, production and other loans to farmers | 30-59 Days Past Due                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 0             |
| Agricultural land, production and other loans to farmers | 60-89 Days Past Due                              |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 0             |
| Agricultural land, production and other loans to farmers | 90 Days or More Past Due                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 27            | 54            |
| Construction                                                                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 949,918       | 835,582       |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Construction | Current                                                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 947,715       | 832,716       |
| Construction | 30-59 Days Past Due                                                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 1,898         | 2,436         |
| Construction | 60-89 Days Past Due                                                                          |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 418           |
| Construction | 90 Days or More Past Due                                                                     |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 305           | 12            |
| Commercial real estate, non-owner occupied                                                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 2,379,819     | 2,407,475     |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 264           |
| Commercial real estate, non-owner occupied | Current                                                        |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 2,362,561     | 2,395,495     |
| Commercial real estate, non-owner occupied | 30-59 Days Past Due                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 6,475         | 5,946         |
| Commercial real estate, non-owner occupied | 60-89 Days Past Due                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 881           |
| Commercial real estate, non-owner occupied | 90 Days or More Past Due                                       |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 10,783        | 5,153         |
| Commercial real estate, owner occupied                                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 1,179,739     | 1,246,528     |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Commercial real estate, owner occupied | Current                                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 1,171,933     | 1,241,714     |
| Commercial real estate, owner occupied | 30-59 Days Past Due                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 7,793         | 4,495         |
| Commercial real estate, owner occupied | 60-89 Days Past Due                                                |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 13            | 0             |
| Commercial real estate, owner occupied | 90 Days or More Past Due                                           |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 319           |
| Residential                                                                                                 |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 2,248,473     | 2,096,655     |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Residential | Current                                                                                       |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 2,228,923     | 2,079,959     |
| Residential | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 10,033        | 8,607         |
| Residential | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 3,657         | 2,278         |
| Residential | 90 Days or More Past Due                                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 5,860         | 5,811         |
| Home equity                                                                                                 |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 614,366       | 630,632       |
| Loans > 90 Days or More Past Due And Accruing                                                            | 150           | 326           |
| Home equity | Current                                                                                       |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 607,419       | 624,543       |
| Home equity | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 3,618         | 2,206         |
| Home equity | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 1,055         | 1,782         |
| Home equity | 90 Days or More Past Due                                                                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 2,274         | 2,101         |
| Individuals' loans for household and other personal expenditures                                            |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 172,896       | 175,211       |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Individuals' loans for household and other personal expenditures | Current                                  |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 171,575       | 174,629       |
| Individuals' loans for household and other personal expenditures | 30-59 Days Past Due                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 878           | 343           |
| Individuals' loans for household and other personal expenditures | 60-89 Days Past Due                      |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 443           | 142           |
| Individuals' loans for household and other personal expenditures | 90 Days or More Past Due                 |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 97            |
| Public finance and other commercial loans                                                                   |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 963,624       | 932,892       |
| Loans > 90 Days or More Past Due And Accruing                                                            | 0             | 0             |
| Public finance and other commercial loans | Current                                                         |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 963,624       | 932,778       |
| Public finance and other commercial loans | 30-59 Days Past Due                                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 114           |
| Public finance and other commercial loans | 60-89 Days Past Due                                             |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | 0             | 0             |
| Public finance and other commercial loans | 90 Days or More Past Due                                        |          |          |
| **Financing Receivable, Past Due [Line Items]**                                                             |          |          |
| Loans                                                                                                       | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479344/326-20-45-1Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3B -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-3BReference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-5
+ Details**Name:**us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost of financing receivable 90 days or more past due and still accruing. Excludes net investment in lease.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 80 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479294/326-20-55-80Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 16 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-16Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Subparagraph (b) -SubTopic 10 -Topic 310 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-7
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-14Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 80 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479294/326-20-55-80
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetNotPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| 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_CommercialRealEstateNonOwnerOccupiedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialRealEstateOwnerOccupiedMember**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_HomeEquityLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_PublicFinanceandOtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |