| **Share-Based Compensation - Components of Share Based Compensation Awards (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                      |
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| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                     |                                                                                                                                                                                                |   |
| Pre-tax compensation expense                                                                                                           | $ 517us-gaap_AllocatedShareBasedCompensationExpense                                                                                                                                                 | $ 505us-gaap_AllocatedShareBasedCompensationExpense |
| Income tax expense (benefit)                                                                                                           | (169)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense                                                                                                                 | (164)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense |
| Total share-based compensation expense, net of income taxes                                                                            | 348us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax                                                                                                                                           | 341us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax |
| Stock and ESPP Options                                                                                                                 |                                                                                                                                                                                                |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                     |                                                                                                                                                                                                |   |
| Pre-tax compensation expense                                                                                                           | 27us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember                                  | 44us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember |
| Income tax expense (benefit)                                                                                                           | (1)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember | (3)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember |
| Total share-based compensation expense, net of income taxes                                                                            | 26us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember                          | 41us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember |
| Restricted Stock Awards                                                                                                                |                                                                                                                                                                                                |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                     |                                                                                                                                                                                                |   |
| Pre-tax compensation expense                                                                                                           | 490us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                                                    | 461us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |
| Income tax expense (benefit)                                                                                                           | (168)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                  | (161)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |
| Total share-based compensation expense, net of income taxes                                                                            | $ 322us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                                          | $ 300us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |

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| - Definition
Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F
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| - Definition
The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.
+ References
No definition available.
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| - Definition
The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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