| **Business Combinations - Assets Acquired and Liabilities Assumed (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015             | Dec. 31, 2014             | Nov. 07, 2014                                                                                                                                                                                              |
|-------------------------------------------------------------------------------------------------------------------------------|---------------------------|---------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Business Acquisition [Line Items]**                                                                                         |                      |                      |                                                                                                                                                                                                       |
| Goodwill                                                                                                                      | $ 202,724us-gaap_Goodwill | $ 202,724us-gaap_Goodwill |                                                                                                                                                                                                       |
| Merger with Community Bancshares, Inc.                                                                                        |                      |                      |                                                                                                                                                                                                       |
| **Business Acquisition [Line Items]**                                                                                         |                      |                      |                                                                                                                                                                                                       |
| Cash and cash equivalents                                                                                                     |                      |                      | 4,124us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                 |
| Interest-bearing time deposits                                                                                                |                      |                      | 16,526frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedInterestBearingTimeDeposits/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member          |
| Investment Securities, available for sale                                                                                     |                      |                      | 76,807us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |
| Loans                                                                                                                         |                      |                      | 145,064frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoansReceivable/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                     |
| Premises and equipment                                                                                                        |                      |                      | 3,610us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member          |
| Federal Home Loan Bank stock                                                                                                  |                      |                      | 1,950frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFederalHomeLoanBankStock/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member              |
| Interest receivable                                                                                                           |                      |                      | 767frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedInterestReceivable/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                      |
| Cash surrender value of life insurance                                                                                        |                      |                      | 3,266frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCashSurrenderValueofLifeInsurance/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member     |
| Other real estate owned                                                                                                       |                      |                      | 6,662frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedOtherRealEstateOwned/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                  |
| Tax asset, deferred and receivable                                                                                            |                      |                      | 3,348us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssetsNoncurrent/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member        |
| Other assets                                                                                                                  |                      |                      | 167us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                |
| Deposits                                                                                                                      |                      |                      | (228,424)frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAssumedDeposits/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |
| Interest payable                                                                                                              |                      |                      | (98)frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedCurrentLiabilitiesInterestPayable/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member      |
| Other liabilities                                                                                                             |                      |                      | (3,014)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member       |
| Net tangible assets acquired                                                                                                  |                      |                      | 30,755us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                               |
| Core deposit intangible                                                                                                       |                      |                      | 4,658us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                        |
| Goodwill                                                                                                                      |                      |                      | 13,776us-gaap_Goodwill/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                                      |
| Purchase price                                                                                                                |                      |                      | $ 49,189us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                     |

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| X                                                                                                                                                                                                                                                                                                                                                                                                             |
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| X                                                                                                                                                                                                                                                                                                                                                                                                    |
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Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Real Estate Owned
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.
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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.
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| - Definition
Amount of deferred tax asset attributable to deductible temporary differences and carryforwards that are expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.
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| - Definition
The amount of identifiable intangible assets recognized as of the acquisition date.
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| - Definition
Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.
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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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
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| - Definition
The amount of property, plant, and equipment recognized as of the acquisition date.
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| - Definition
Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.
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| - Definition
Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
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