| **Purchase and Assumption - Assets and Liabilities of SCB Recorded at Provisional Fair Value (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended | 0 Months Ended                                                      |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|---------------------------------------------------------------------|
||
| **Assets**                                                                                                                                              |            |                                                                |   |
| Investment securities, available for sale                                                                                                               |            |                                                                | $ 18,900us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities |
| Federal Home Loan Bank stock                                                                                                                            |            |                                                                | 1,800frme_BusinessAcquisitionPurchasePriceAllocationFederalHomeLoanBankStock |
| **Loans:**                                                                                                                                              |            |                                                                |   |
| Total Assets Purchased                                                                                                                                  |            |                                                                | 147,700us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Liabilities Assumed                                                                                                                               |            |                                                                | 135,700us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities |
| Net Gain on Acquisition                                                                                                                                 |       | 9,124us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount |   |
| Cash received from FDIC                                                                                                                                 |            |                                                                | 17,200frme_BusinessAcquisitionPurchasePriceAllocationCashReceivedFromFederalDepositInsuranceCorporation |
| SCB Bank                                                                                                                                                |            |                                                                |   |
| **Assets**                                                                                                                                              |            |                                                                |   |
| Cash and due from banks                                                                                                                                 |            |                                                                | 29,113frme_BusinessAcquisitionPurchasePriceAllocationCashAndDueFromBanks/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Investment securities, available for sale                                                                                                               |            |                                                                | 18,896us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Federal Home Loan Bank stock                                                                                                                            |            |                                                                | 1,761frme_BusinessAcquisitionPurchasePriceAllocationFederalHomeLoanBankStock/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| **Loans:**                                                                                                                                              |            |                                                                |   |
| Total Loans                                                                                                                                             |            |                                                                | 93,793frme_BusinessAcquisitionPurchasePriceAllocationLoansReceivable/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Premises                                                                                                                                                |            |                                                                | 1,516us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Core deposit intangible                                                                                                                                 |            |                                                                | 484us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Other assets                                                                                                                                            |            |                                                                | 560us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Total Assets Purchased                                                                                                                                  |            |                                                                | 146,123us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Deposits                                                                                                                                          |            |                                                                | 125,909frme_BusinessAcquisitionPurchasePriceAllocationLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Federal Home Loan Bank advances                                                                                                                         |            |                                                                | 10,286frme_BusinessAcquisitionPurchasePriceAllocationFederalHomeLoanBankAdvances/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Other liabilities                                                                                                                                       |            |                                                                | 804us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Total Liabilities Assumed                                                                                                                               |            |                                                                | 136,999us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| Net Gain on Acquisition                                                                                                                                 |            |                                                                | 9,124us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| SCB Bank | Non-interest bearing                                                                                                                         |            |                                                                |   |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Deposits                                                                                                                                          |            |                                                                | 13,715frme_BusinessAcquisitionPurchasePriceAllocationLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FairValueByLiabilityClassAxis= frme_NonInterestBearingDepositsMember |
| SCB Bank | NOW accounts                                                                                                                                 |            |                                                                |   |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Deposits                                                                                                                                          |            |                                                                | 14,746frme_BusinessAcquisitionPurchasePriceAllocationLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FairValueByLiabilityClassAxis= frme_NegotiableOrderOfWithdrawalAccountsMember |
| SCB Bank | Savings and money market                                                                                                                     |            |                                                                |   |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Deposits                                                                                                                                          |            |                                                                | 25,843frme_BusinessAcquisitionPurchasePriceAllocationLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FairValueByLiabilityClassAxis= us-gaap_MoneyMarketFundsMember |
| SCB Bank | Certificate of deposit                                                                                                                       |            |                                                                |   |
| **Deposits:**                                                                                                                                           |            |                                                                |   |
| Total Deposits                                                                                                                                          |            |                                                                | 71,605frme_BusinessAcquisitionPurchasePriceAllocationLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember/ us-gaap_FairValueByLiabilityClassAxis= us-gaap_CertificatesOfDepositMember |
| SCB Bank | Commercial                                                                                                                                   |            |                                                                |   |
| **Loans:**                                                                                                                                              |            |                                                                |   |
| Total Loans                                                                                                                                             |            |                                                                | 51,042frme_BusinessAcquisitionPurchasePriceAllocationLoansReceivable/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis= us-gaap_CommercialLoanMember/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| SCB Bank | Residential mortgage                                                                                                                         |            |                                                                |   |
| **Loans:**                                                                                                                                              |            |                                                                |   |
| Total Loans                                                                                                                                             |            |                                                                | 11,181frme_BusinessAcquisitionPurchasePriceAllocationLoansReceivable/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis= us-gaap_ResidentialMortgageMember/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |
| SCB Bank | Installment                                                                                                                                  |            |                                                                |   |
| **Loans:**                                                                                                                                              |            |                                                                |   |
| Total Loans                                                                                                                                             |            |                                                                | $ 31,570frme_BusinessAcquisitionPurchasePriceAllocationLoansReceivable/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis= frme_InstallmentLoansMember/ us-gaap_BusinessAcquisitionAxis= frme_ScbBankMember |

| X                                                                                                                                                                                                                                                                                                                           |
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| - Definition
Business Acquisition Purchase Price Allocation Cash Received From Federal Deposit Insurance Corporation
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| - Definition
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| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Purchase Price Allocation, Loans Receivable
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
In a business combination in which the amount of net identifiable assets acquired and liabilities assumed exceeds the aggregate consideration transferred or to be transferred (as defined), this element represents the amount of gain recognized by the entity.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of assets acquired at the acquisition date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of facility held for productive use including, but not limited to, office, production, storage and distribution facilities, acquired at the acquisition date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of identifiable intangible assets recognized as of the acquisition date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of liabilities assumed at the acquisition date.
+ References
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| - Definition
Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.
+ References
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| X                                                                                                                                          |
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