| **Disclosures About Fair Value of Assets and Liabilities - Valuation Methodologies Used for Instruments Measured at Fair Value on Non-Recurring Basis (Details) (Fair Value, Measurements, Nonrecurring, USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                                                                 | Dec. 31, 2014                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Impaired loans (collateral dependent)                                                                                                                                                                                                                     |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                          |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| Fair Value                                                                                                                                                                                                                                                | $ 7,485us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                           | $ 17,134us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                            |
| Other real estate owned                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                          |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| Fair Value                                                                                                                                                                                                                                                | 663us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                        | 5,155us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                        |
| Significant Unobservable Inputs (Level 3) | Impaired loans (collateral dependent)                                                                                                                                                                         |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                          |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| Fair Value                                                                                                                                                                                                                                                | 7,485us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember        | 17,134us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_ImpairedLoansMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember         |
| Significant Unobservable Inputs (Level 3) | Other real estate owned                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                          |                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                            |
| Fair Value                                                                                                                                                                                                                                                | $ 663us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember | $ 5,155us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= frme_OtherRealEstateOwnedMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |