| **Share-Based Compensation - Components of Awards (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                           |                                                                                                                                                                                                 |   |   |
| Pre-tax compensation expense                                                                                 | $ 2,177us-gaap_AllocatedShareBasedCompensationExpense                                                                                                                                                | $ 1,773us-gaap_AllocatedShareBasedCompensationExpense | $ 1,492us-gaap_AllocatedShareBasedCompensationExpense |
| Income tax benefit                                                                                           | (726)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense                                                                                                                  | (547)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | (451)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense |
| Total share-based compensation expense, net of income taxes                                                  | 1,451us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax                                                                                                                                          | 1,226us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax | 1,041us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax |
| Stock and ESPP Options                                                                                       |                                                                                                                                                                                                 |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                           |                                                                                                                                                                                                 |   |   |
| Pre-tax compensation expense                                                                                 | 237us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember                                  | 253us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember | 284us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember |
| Income tax benefit                                                                                           | (47)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember | (16)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember | (23)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember |
| Total share-based compensation expense, net of income taxes                                                  | 190us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember                          | 237us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_StockOptionsAndEmployeeStockPurchasePlanMember | 261us-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                                                                                 | 1,940us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                                                   | 1,520us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember | 1,208us-gaap_AllocatedShareBasedCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |
| Income tax benefit                                                                                           | (679)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                   | (531)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember | (428)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |
| Total share-based compensation expense, net of income taxes                                                  | $ 1,261us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember                                         | $ 989us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember | $ 780us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis= frme_RestrictedStockAwardsMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
+ Details**Name:**us-gaap_AllocatedShareBasedCompensationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ Details**Name:**us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
+ Details**Name:**us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis=frme_RestrictedStockAwardsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis=frme_StockOptionsAndEmployeeStockPurchasePlanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |