| **Loans and Allowance - Additional Information (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                    |                                                                                                                                                                     |                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                           |                                                                                                                                                                                                                              |                                                                                                                                                                |                                                                                                                                                                             |   |
| Loans held for sale                                                                                        | $ 6,392us-gaap_LoansReceivableHeldForSaleNet                                                                                                                                                                                      |                                                                                                                                                                | $ 7,235us-gaap_LoansReceivableHeldForSaleNet                                                                                                                                     |   |
| Loans - allowance for loan losses                                                                          | 62,801us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                                                                   | 69,583us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                     | 63,964us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                                  | 67,870us-gaap_LoansAndLeasesReceivableAllowance |
| Decrease in allowance for loan losses                                                                      | 1,163us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease                                                                                                                                                                 |                                                                                                                                                                |                                                                                                                                                                             |   |
| Allowance for impaired financing receivables change                                                        | 2,762frme_ImpairedFinancingReceivableRelatedAllowancePeriodIncreaseDecrease                                                                                                                                                       |                                                                                                                                                                |                                                                                                                                                                             |   |
| Impaired financing receivable, allowance                                                                   | 4,556us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                                                                          | 1,794us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                            | 2,769us-gaap_ImpairedFinancingReceivableRelatedAllowance                                                                                                                         |   |
| Allowance for loan losses, write-offs (recoveries)                                                         | 1,163us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet                                                                                                                                                                           | (1,713)us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet                                                                                                           |                                                                                                                                                                             |   |
| Real estate loans - Residential                                                                            |                                                                                                                                                                                                                              |                                                                                                                                                                |                                                                                                                                                                             |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                           |                                                                                                                                                                                                                              |                                                                                                                                                                |                                                                                                                                                                             |   |
| Loans - allowance for loan losses                                                                          | 13,269us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                               | 13,908us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember | 13,152us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember              | 15,077us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Impaired financing receivable, allowance                                                                   | 554us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember                                                        |                                                                                                                                                                | 194us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember       |   |
| Real estate loans - Commercial and farmland                                                                |                                                                                                                                                                                                                              |                                                                                                                                                                |                                                                                                                                                                             |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                           |                                                                                                                                                                                                                              |                                                                                                                                                                |                                                                                                                                                                             |   |
| Impaired financing receivable, allowance                                                                   | $ 1,302us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember                                                  |                                                                                                                                                                | $ 1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |   |
| Percentage of troubled debt restructured loans                                                             | 54.00%frme_FinancingReceivableModificationsPostModificationLoansAsPercentageOfTroubledDebtRestructuredLoans/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= frme_CommercialAndFarmLandLoansMember |                                                                                                                                                                |                                                                                                                                                                             |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Post-Modification, Loans as Percentage of Troubled Debt Restructured Loans
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableModificationsPostModificationLoansAsPercentageOfTroubledDebtRestructuredLoans**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired Financing Receivable Related Allowance, Period Increase (Decrease)
+ References
No definition available.
+ Details**Name:**frme_ImpairedFinancingReceivableRelatedAllowancePeriodIncreaseDecrease**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) of allowance reserve for credit loss.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9
+ Details**Name:**us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after recoveries, of loans and leases that have been written-off.
+ References
No definition available.
+ Details**Name:**us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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=35738203&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 allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section L -Subsection 1
+ Details**Name:**us-gaap_LoansAndLeasesReceivableAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph a -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 35 -Paragraph 49 -URI http://asc.fasb.org/extlink&oid=35742846&loc=d3e3084-111518Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6490209&loc=d3e48214-111002
+ Details**Name:**us-gaap_LoansReceivableHeldForSaleNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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:** |