| **Loans and Allowance - Changes in Allowance for Loan Losses by Loan Segment (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                              |
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| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | $ 63,964us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                           | $ 67,870us-gaap_LoansAndLeasesReceivableAllowance |
| Provision for losses                                                                                                                     |                                                                                                                                                                   |    |
| Recoveries on loans                                                                                                                      | 1,072frme_AllowanceForLoanAndLeaseLossesLoanRecoveries                                                                                                                      | 3,600frme_AllowanceForLoanAndLeaseLossesLoanRecoveries |
| Loans charged off                                                                                                                        | (2,235)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs                                                                                                                      | (1,887)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs |
| Ending balance                                                                                                                           | 62,801us-gaap_LoansAndLeasesReceivableAllowance                                                                                                                             | 69,583us-gaap_LoansAndLeasesReceivableAllowance |
| Commercial                                                                                                                               |                                                                                                                                                                        |   |
| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | 28,824us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                | 27,176us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember |
| Provision for losses                                                                                                                     | 1,834us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember               | 2,387us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember |
| Recoveries on loans                                                                                                                      | 450frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember           | 2,050frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember |
| Loans charged off                                                                                                                        | (1,101)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember         | (706)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember |
| Ending balance                                                                                                                           | 30,007us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember                | 30,907us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialLoanMember |
| Commercial Real Estate                                                                                                                   |                                                                                                                                                                        |   |
| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | 19,327us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember          | 23,102us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember |
| Provision for losses                                                                                                                     | (2,896)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember       | (1,257)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember |
| Recoveries on loans                                                                                                                      | 412frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember     | 790frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember |
| Loans charged off                                                                                                                        | (460)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember     | (277)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember |
| Ending balance                                                                                                                           | 16,383us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember          | 22,358us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_CommercialRealEstateMember |
| Consumer                                                                                                                                 |                                                                                                                                                                        |   |
| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | 2,658us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                   | 2,515us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Provision for losses                                                                                                                     | 527us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                   | (12)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Recoveries on loans                                                                                                                      | 78frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember              | 136frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Loans charged off                                                                                                                        | (125)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember             | (229)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Ending balance                                                                                                                           | 3,138us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember                   | 2,410us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ConsumerLoanMember |
| Residential                                                                                                                              |                                                                                                                                                                        |   |
| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | 13,152us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         | 15,077us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Provision for losses                                                                                                                     | 534us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember          | (1,098)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Recoveries on loans                                                                                                                      | 132frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | 604frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Loans charged off                                                                                                                        | (549)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember    | (675)us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Ending balance                                                                                                                           | 13,269us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember         | 13,908us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_ResidentialRealEstateMember |
| Finance Leases                                                                                                                           |                                                                                                                                                                        |   |
| **Allowance for loan losses:**                                                                                                           |                                                                                                                                                                        |   |
| Beginning balance                                                                                                                        | 3us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember   |    |
| Provision for losses                                                                                                                     | 1us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember | (20)us-gaap_ProvisionForLoanLeaseAndOtherLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |
| Recoveries on loans                                                                                                                      |                                                                                                                                                                   | 20frme_AllowanceForLoanAndLeaseLossesLoanRecoveries/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |
| Loans charged off                                                                                                                        |                                                                                                                                                                   |    |
| Ending balance                                                                                                                           | $ 4us-gaap_LoansAndLeasesReceivableAllowance/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_FinanceLeasesFinancingReceivableMember |    |

| X                                                                                                                                                                                                                                                                                        |
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| - Definition
Allowance for Loan and Lease Losses, Loan Recoveries
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of loans and leases that have been written off from both loan receivables and 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 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
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| - 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| - Definition
Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 9Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524
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| X                                                                                                                                                                                                  |
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| X                                                                                                                                                                                                        |
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| X                                                                                                                                                                                                |
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| X                                                                                                                                                                                                                    |
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| X                                                                                                                                                                                                         |
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