| **Disclosures About Fair Value of Assets and Liabilities - Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements Other Than Goodwill (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Thousands** | 3 Months Ended | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| State and municipal | Discounted cash flow                                                                                                                                                                                                             |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Fair Value                                                                                                                                                                                                                                             | $ 5,469        | $ 5,897         |
| State and municipal | Discounted cash flow | 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                                                                                                                                                                                                                                          | 0.69%          | 0.80%           |
| State and municipal | Discounted cash flow | 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%          | 5.00%           |
| Corporate obligations and Equity securities | Discounted cash flow                                                                                                                                                                                     |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Fair Value                                                                                                                                                                                                                                             | $ 35           | $ 35            |
| Risk free rate                                                                                                                                                                                                                                         | 3 month LIBOR  | 3 month LIBOR   |
| Discount to reflect current market conditions                                                                                                                                                                                                          | 2.00%          | 2.00%           |
| Impaired loans (collateral dependent) | Collateral based measurements                                                                                                                                                                                  |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Fair Value                                                                                                                                                                                                                                             | $ 9,530        | $ 7,066         |
| Impaired loans (collateral dependent) | Collateral based measurements | Minimum                                                                                                                                                                        |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                                              | 0.00%          | 0.00%           |
| Impaired loans (collateral dependent) | Collateral based measurements | Maximum                                                                                                                                                                        |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                                              | 50.00%         | 50.00%          |
| Impaired loans (collateral dependent) | Collateral based measurements | Weighted Average                                                                                                                                                               |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                                                              | 1.00%          | 2.00%           |
| Other real estate owned | Appraisals                                                                                                                                                                                                                   |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Fair Value                                                                                                                                                                                                                                             | $ 1,287        | $ 5,529         |
| Other real estate owned | Appraisals | Minimum                                                                                                                                                                                                         |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions                                                                                                                                                                                                          | 0.00%          | 0.00%           |
| Other real estate owned | Appraisals | Maximum                                                                                                                                                                                                         |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions                                                                                                                                                                                                          | 20.00%         | 20.00%          |
| Other real estate owned | Appraisals | Weighted Average                                                                                                                                                                                                |           |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                                                             |           |            |
| Discount to reflect current market conditions                                                                                                                                                                                                          | 2.00%          | 2.00%           |

| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Discount Rate Description
+ References
No definition available.
+ Details**Name:**frme_FairValueInputsDiscountRateDescription**Namespace Prefix:**frme_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Maturity
+ References
No definition available.
+ Details**Name:**frme_SecuritiesMaturity**Namespace Prefix:**frme_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
+ Details**Name:**us-gaap_DebtInstrumentCreditRating**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.
+ 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 (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsDiscountForLackOfMarketability**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsDiscountRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.
+ 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 (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsPrepaymentRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ValuationTechniqueAxis=frme_DiscountedCashFlowMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=frme_CorporateDebtSecuritiesandEquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=frme_ImpairedLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ValuationTechniqueAxis=frme_CollateralBasedMeasurementsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_WeightedAverageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=frme_OtherRealEstateOwnedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ValuationTechniqueAxis=frme_AppraisalsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |