| **Income Tax - Narrative (Details) (USD $)In Thousands, unless otherwise specified**                                      | 12 Months Ended                                                                                                                                      |
|---------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Tax expense applicable to security gains and losses, including unrealized losses relating to other-than-temporary charges | $ 760frme_MarketableSecuritiesGainLossIncludingOtherThanTemporaryImpairmentsTax                                                                      | $ 157frme_MarketableSecuritiesGainLossIncludingOtherThanTemporaryImpairmentsTax | $ 759frme_MarketableSecuritiesGainLossIncludingOtherThanTemporaryImpairmentsTax |
| Increase (decrease) in deferred tax assets                                                                                | 16,393frme_IncreaseDecreaseinDeferredTaxAssets                                                                                                       |   |   |
| Valuation allowance                                                                                                       | 17,568us-gaap_DeferredTaxAssetsValuationAllowance                                                                                                    | 17,171us-gaap_DeferredTaxAssetsValuationAllowance | 13,859us-gaap_DeferredTaxAssetsValuationAllowance |
| Additional paid-in capital                                                                                                | 431,220us-gaap_AdditionalPaidInCapital                                                                                                               | 393,783us-gaap_AdditionalPaidInCapital |   |
| Unrecorded deferred tax liability                                                                                         | 4,688us-gaap_DeferredTaxLiabilityNotRecognizedAmountOfUnrecognizedDeferredTaxLiabilityUndistributedEarningsOfDomesticSubsidiaries                    |   |   |
| Merger with CFS Bancorp, Inc. (Citizens)                                                                                  |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Additional paid-in capital                                                                                                | 13,393us-gaap_AdditionalPaidInCapital/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                  |   |   |
| State and Local Jurisdiction                                                                                              |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Valuation allowance                                                                                                       | 17,568us-gaap_DeferredTaxAssetsValuationAllowance/ us-gaap_IncomeTaxAuthorityAxis= us-gaap_StateAndLocalJurisdictionMember                           |   |   |
| State tax loss carryforward                                                                                               | 143,467us-gaap_OperatingLossCarryforwards/ us-gaap_IncomeTaxAuthorityAxis= us-gaap_StateAndLocalJurisdictionMember                                   |   |   |
| Valuation allowance on state loss carryforward                                                                            | 142,358us-gaap_OperatingLossCarryforwardsValuationAllowance/ us-gaap_IncomeTaxAuthorityAxis= us-gaap_StateAndLocalJurisdictionMember                 |   |   |
| Available-for-sale securities                                                                                             |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Increase in deferred tax liability                                                                                        | 7,742frme_IncreaseDecreaseinDeferredTaxLiability/ invest_InvestmentAxis= us-gaap_AvailableforsaleSecuritiesMember                                    |   |   |
| Deferred Compensation                                                                                                     |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Increase (decrease) in deferred tax assets                                                                                | (2,180)frme_IncreaseDecreaseinDeferredTaxAssets/ us-gaap_StatementEquityComponentsAxis= us-gaap_DeferredCompensationShareBasedPaymentsMember         |   |   |
| Loans                                                                                                                     |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Increase (decrease) in deferred tax assets                                                                                | 3,669frme_IncreaseDecreaseinDeferredTaxAssets/ us-gaap_ValuationAllowancesAndReservesTypeAxis= us-gaap_AllowanceForLoansAndLeasesReceivableMember    |   |   |
| Other than temporary impairment on securities                                                                             |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Increase (decrease) in deferred tax assets                                                                                | 3,261frme_IncreaseDecreaseinDeferredTaxAssets/ us-gaap_ValuationAllowancesAndReservesTypeAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |   |   |
| Other real estate owned                                                                                                   |                                                                                                                                                 |   |   |
| **Valuation Allowance [Line Items]**                                                                                      |                                                                                                                                                 |   |   |
| Increase (decrease) in deferred tax assets                                                                                | $ 1,988frme_IncreaseDecreaseinDeferredTaxAssets/ us-gaap_ValuationAllowancesAndReservesTypeAxis= us-gaap_ValuationAllowanceRealEstateOwnedMember     |   |   |

| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Increase (Decrease) in Deferred Tax Assets
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Increase (Decrease) in Deferred Tax Liability
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Marketable Securities Gain (Loss) Including Other Than Temporary Impairments, Tax
+ References
No definition available.
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| - Definition
Excess of issue price over par or stated value of the entity's capital stock and amounts received from other transactions involving the entity's stock or stockholders. Includes adjustments to additional paid in capital. Some examples of such adjustments include recording the issuance of debt with a beneficial conversion feature and certain tax consequences of equity instruments awarded to employees. Use this element for the aggregate amount of additional paid-in capital associated with common and preferred stock. For additional paid-in capital associated with only common stock, use the element additional paid in capital, common stock. For additional paid-in capital associated with only preferred stock, use the element additional paid in capital, preferred stock.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.30(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax liability not recognized because of the exceptions to comprehensive recognition of deferred taxes related to undistributed earnings of domestic subsidiaries.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of the valuation allowance pertaining to the deferred tax asset representing potential future taxable deductions from net operating loss carryforwards for which it is more likely than not that a tax benefit will not be realized.
+ References
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