| **Disclosures About Fair Value of Assets and Liabilities - Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements Other Than Goodwill (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                                                                    | 12 Months Ended                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Discounted cash flow | State and municipal | Minimum                                                                                                                                                                              |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Maturity/Call date                                                                                                                                                                                                                | 1 month                                                                                                                                                                                                                                                                           | 1 month                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                              |
| Blend of US Muni BQ curve                                                                                                                                                                                                         | A-                                                                                                                                                                                                                                                                                | A-                                                                                                                                                                                                                                      |                                                                                                                                                                                                                                                                              |
| Discount rate                                                                                                                                                                                                                     | 0.90%us-gaap_FairValueInputsDiscountRate/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember                                              | 0.90%us-gaap_FairValueInputsDiscountRate/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember    |                                                                                                                                                                                                                                                                              |
| Discounted cash flow | State and municipal | Maximum                                                                                                                                                                              |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Maturity/Call date                                                                                                                                                                                                                | 15 years                                                                                                                                                                                                                                                                          | 15 years                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                              |
| Blend of US Muni BQ curve                                                                                                                                                                                                         | BBB-                                                                                                                                                                                                                                                                              | BBB-                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount rate                                                                                                                                                                                                                     | 5.00%us-gaap_FairValueInputsDiscountRate/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember                                              | 5.00%us-gaap_FairValueInputsDiscountRate/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember    |                                                                                                                                                                                                                                                                              |
| Discounted cash flow | Corporate obligations and Equity securities                                                                                                                                                                |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Risk free rate                                                                                                                                                                                                                    | 3 month LIBOR                                                                                                                                                                                                                                                                     | 3 month LIBOR                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions                                                                                                                                                                                     | 2.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_CorporateDebtSecuritiesandEquitySecuritiesMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember                                                               | 2.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_CorporateDebtSecuritiesandEquitySecuritiesMember/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember                     |                                                                                                                                                                                                                                                                              |
| Collateral based measurements | Impaired loans (collateral dependent) | Minimum                                                                                                                                                   |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                         | 0.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember                                                         | 0.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember               |                                                                                                                                                                                                                                                                              |
| Collateral based measurements | Impaired loans (collateral dependent) | Maximum                                                                                                                                                   |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                         | 50.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember                                                        | 50.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember              |                                                                                                                                                                                                                                                                              |
| Collateral based measurements | Impaired loans (collateral dependent) | Weighted Average                                                                                                                                          |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                         | 4.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember                                                 | 3.00%us-gaap_FairValueInputsPrepaymentRate/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember       |                                                                                                                                                                                                                                                                              |
| Appraisals | Other real estate owned | Minimum                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions                                                                                                                                                                                     | 0.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember                                                   | 0.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember         |                                                                                                                                                                                                                                                                              |
| Appraisals | Other real estate owned | Maximum                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions                                                                                                                                                                                     | 20.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember                                                  | 20.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember        |                                                                                                                                                                                                                                                                              |
| Appraisals | Other real estate owned | Weighted Average                                                                                                                                                                           |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Discount to reflect current market conditions                                                                                                                                                                                     | 1.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember                                           | 7.00%us-gaap_FairValueInputsDiscountForLackOfMarketability/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember |                                                                                                                                                                                                                                                                              |
| Significant Unobservable Inputs (Level 3) | Discounted cash flow | State and municipal                                                                                                                                            |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Fair Value                                                                                                                                                                                                                        | 6,163us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember     |                                                                                                                                                                                                                                    | $ 6,611us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember   |
| Significant Unobservable Inputs (Level 3) | Discounted cash flow | Corporate obligations and Equity securities                                                                                                                    |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Fair Value                                                                                                                                                                                                                        | 35us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_CorporateDebtSecuritiesandEquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember |                                                                                                                                                                                                                                    | 35us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_CorporateDebtSecuritiesandEquitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_DiscountedCashFlowMember |
| Significant Unobservable Inputs (Level 3) | Collateral based measurements | Impaired loans (collateral dependent)                                                                                                                 |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Fair Value                                                                                                                                                                                                                        | 7,485us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember                  |                                                                                                                                                                                                                                    | 17,134us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_CollateralBasedMeasurementsMember                 |
| Significant Unobservable Inputs (Level 3) | Appraisals | Other real estate owned                                                                                                                                                  |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                        |                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                              |
| Fair Value                                                                                                                                                                                                                        | 663us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember                              |                                                                                                                                                                                                                                    | $ 5,155us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_ValuationTechniqueAxis= frme_AppraisalsMember                          |

| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Discount Rate Description
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No definition available.
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| - Definition
Securities, Maturity
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5
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| - Definition
Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.
+ References
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| - Definition
Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.  For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.
+ References
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