| **Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands** | 12 Months Ended |
|----------------------------------------------------------------------------------------------------------|-----------------|
||
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 134,581       | $ 158,563 | $ 189,779 |
| Impaired loans with related allowance                                                                    | 7,953           | 16,703 | 6,121 |
| Total Impaired Loans                                                                                     | 142,534         | 175,266 | 195,900 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 90,975          | 102,793 | 114,253 |
| Impaired loans with related allowance                                                                    | 6,552           | 13,430 | 5,502 |
| Total Impaired Loans                                                                                     | 97,527          | 116,223 | 119,755 |
| Related Allowance                                                                                        | 1,842           | 2,769 | 1,583 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 96,423          | 106,807 | 119,561 |
| Impaired loans with related allowance                                                                    | 6,945           | 15,764 | 5,607 |
| Total Impaired Loans                                                                                     | 103,368         | 122,571 | 125,168 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 | 4,217           | 4,297 | 2,048 |
| Impaired loans with related allowance                                                                    | 38              | 65 | 18 |
| Total Impaired Loans                                                                                     | 4,255           | 4,362 | 2,066 |
| Commercial and industrial loans                                                                          |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | 22,151          | 35,514 | 35,066 |
| Impaired loans with related allowance                                                                    | 3,043           | 1,766 | 1,390 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 11,669          | 18,029 | 16,371 |
| Impaired loans with related allowance                                                                    | 2,690           | 1,684 | 1,216 |
| Related Allowance                                                                                        | 1,247           | 1,055 | 683 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 12,578          | 18,711 | 19,209 |
| Impaired loans with related allowance                                                                    | 2,752           | 1,721 | 1,240 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 | 488             | 362 | 192 |
| Impaired loans with related allowance                                                                    | 38              | 40 | 9 |
| Agriculture production financing and other loans to farmers                                              |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | 370             | 26 | 32 |
| Impaired loans with related allowance                                                                    | 466             | 6,777 |   |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 361             | 22 | 30 |
| Impaired loans with related allowance                                                                    | 466             | 5,777 |   |
| Related Allowance                                                                                        | 30              | 400 |   |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 439             | 26 | 32 |
| Impaired loans with related allowance                                                                    | $ 538           | 8,044 |   |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with related allowance                                                                    |                 | 1 |   |
| Construction                                                                                             |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 4,551         | 12,956 | 16,109 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 2,336           | 9,318 | 10,625 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 3,662           | 9,837 | 11,621 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 | 157             | 427 | 117 |
| Commercial and farmland                                                                                  |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | 95,930          | 95,856 | 128,073 |
| Impaired loans with related allowance                                                                    | 2,144           | 7,159 | 4,657 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 69,024          | 68,187 | 83,033 |
| Impaired loans with related allowance                                                                    | 1,933           | 4,971 | 4,215 |
| Related Allowance                                                                                        | 392             | 1,120 | 894 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 71,569          | 70,844 | 84,057 |
| Impaired loans with related allowance                                                                    | 1,868           | 4,999 | 4,291 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 3,328         | 3,389 | 1,663 |
| Impaired loans with related allowance                                                                    |                 | 24 | 9 |
| Residential                                                                                              |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 11,262        | 10,591 | 6,746 |
| Impaired loans with related allowance                                                                    | 2,300           | 1,001 | 74 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 7,338           | 6,839 | 3,910 |
| Impaired loans with related allowance                                                                    | 1,463           | 998 | 71 |
| Related Allowance                                                                                        | 173             | 194 | 6 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | 7,926           | 6,987 | 4,236 |
| Impaired loans with related allowance                                                                    | 1,787           | 1,000 | 76 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 244           | $ 119 | $ 75 |
| Impaired loans with related allowance                                                                    |                 |  |  |
| Home equity                                                                                              |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 297           | $ 3,590 | $ 3,299 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 | 247             | 398 | 112 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 249           | $ 402 | $ 225 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 |                 |  |  |
| Other commercial loans                                                                                   |            |   |   |
| **Unpaid Principal Balance**                                                                             |            |   |   |
| Impaired loans with no related allowance                                                                 | $ 20            | $ 30 | $ 454 |
| **Recorded Investment**                                                                                  |            |   |   |
| Impaired loans with no related allowance                                                                 |                 |  | 172 |
| **Average Recorded Investment**                                                                          |            |   |   |
| Impaired loans with no related allowance                                                                 |                 |  | 181 |
| **Interest Income Recognized**                                                                           |            |   |   |
| Impaired loans with no related allowance                                                                 |                 |  | $ 1 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ImpairedFinancingReceivableAverageRecordedInvestmentAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethodAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestmentAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for credit losses related to recorded investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRelatedAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalanceAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_OtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |