| **Business Combination - Narrative (Details) (USD $)**                   | 12 Months Ended                                                                                                                             | 0 Months Ended                                                                                                                               |
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| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Value of stock issued as a part of acquisition                           | $ 34,981,000us-gaap_StockIssuedDuringPeriodValueAcquisitions                                                                                | $ 135,642,000us-gaap_StockIssuedDuringPeriodValueAcquisitions                                                                                |   |   |
| Common Stock                                                             |                                                                                                                                        |                                                                                                                                         |   |   |
| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Stock issued as a part of acquisition (in shares)                        | 1,574,298us-gaap_StockIssuedDuringPeriodSharesAcquisitions/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember                | 7,079,457us-gaap_StockIssuedDuringPeriodSharesAcquisitions/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember                 |   |   |
| Value of stock issued as a part of acquisition                           | 197,000us-gaap_StockIssuedDuringPeriodValueAcquisitions/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember                   | 885,000us-gaap_StockIssuedDuringPeriodValueAcquisitions/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember                    |   |   |
| Merger with Community Bancshares, Inc.                                   |                                                                                                                                        |                                                                                                                                         |   |   |
| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Percentage of interest acquired                                          |                                                                                                                                        |                                                                                                                                         | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Number of banking centers acquired                                       |                                                                                                                                        |                                                                                                                                         | 10us-gaap_NumberOfBusinessesAcquired/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Number of shares of common stock                                         |                                                                                                                                        |                                                                                                                                         | 4.0926frme_BusinessCombinationsAcquireeSharesOfStockReceivedForEachShareCurrentlyOwned/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Share price                                                              |                                                                                                                                        |                                                                                                                                         | $ 85.94us-gaap_BusinessAcquisitionSharePrice/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Cash paid in acquisition                                                 |                                                                                                                                        |                                                                                                                                         | 14,200,000us-gaap_PaymentsToAcquireBusinessesGross/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Stock issued as a part of acquisition (in shares)                        |                                                                                                                                        |                                                                                                                                         | 1,600,000us-gaap_StockIssuedDuringPeriodSharesAcquisitions/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Value of stock issued as a part of acquisition                           |                                                                                                                                        |                                                                                                                                         | 35,000,000us-gaap_StockIssuedDuringPeriodValueAcquisitions/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Total purchase price                                                     |                                                                                                                                        |                                                                                                                                         | 49,189,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Core deposit intangible                                                  |                                                                                                                                        |                                                                                                                                         | 4,658,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |
| Acquired intangible asset, expected useful life                          |                                                                                                                                        |                                                                                                                                         | 10 years |   |
| Pro forma revenue                                                        | 1,600,000us-gaap_BusinessAcquisitionsProFormaRevenue/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member         |                                                                                                                                         |   |   |
| Pro forma nonrecurring expense                                           | 1,800,000frme_BusinessAcquisitionProFormaNonrecurringExpense/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |                                                                                                                                         |   |   |
| Merger with Community Bancshares, Inc. | Common Stock                    |                                                                                                                                        |                                                                                                                                         |   |   |
| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Number of shares of common stock                                         |                                                                                                                                        |                                                                                                                                         | 4.0926frme_BusinessCombinationsAcquireeSharesOfStockReceivedForEachShareCurrentlyOwned/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member/ us-gaap_StatementEquityComponentsAxis= us-gaap_CommonStockMember |   |
| Merger with CFS Bancorp, Inc. (Citizens)                                 |                                                                                                                                        |                                                                                                                                         |   |   |
| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Percentage of interest acquired                                          |                                                                                                                                        |                                                                                                                                         |   | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Number of banking centers acquired                                       |                                                                                                                                        |                                                                                                                                         |   | 20us-gaap_NumberOfBusinessesAcquired/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Stock issued as a part of acquisition (in shares)                        |                                                                                                                                        |                                                                                                                                         |   | 7,100,000us-gaap_StockIssuedDuringPeriodSharesAcquisitions/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Value of stock issued as a part of acquisition                           |                                                                                                                                        |                                                                                                                                         |   | 135,600,000us-gaap_StockIssuedDuringPeriodValueAcquisitions/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Total purchase price                                                     |                                                                                                                                        |                                                                                                                                         |   | 135,642,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Core deposit intangible                                                  |                                                                                                                                        |                                                                                                                                         |   | 7,313,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Acquired intangible asset, expected useful life                          |                                                                                                                                        |                                                                                                                                         |   | 10 years |
| Pro forma revenue                                                        |                                                                                                                                        | 4,900,000us-gaap_BusinessAcquisitionsProFormaRevenue/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember           |   |   |
| Pro forma nonrecurring expense                                           |                                                                                                                                        | $ 9,500,000frme_BusinessAcquisitionProFormaNonrecurringExpense/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |   |   |
| Merger with CFS Bancorp, Inc. (Citizens) | CFS Bancorp                   |                                                                                                                                        |                                                                                                                                         |   |   |
| **Business Acquisition [Line Items]**                                    |                                                                                                                                        |                                                                                                                                         |   |   |
| Percentage of common stock received per shares owned through acquisition |                                                                                                                                        |                                                                                                                                         |   | 0.65%frme_BusinessCombinationsAcquireePercentageofCommonStockReceivedForEachShareCurrentlyOwned/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember/ dei_LegalEntityAxis= frme_CFSBancorpMember |

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| - Definition
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| - Definition
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| - Definition
Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
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| - Definition
Percentage of voting equity interests acquired at the acquisition date in the business combination.
+ References
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| - Definition
Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.
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| - Definition
The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.
+ References
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| - Definition
The amount of identifiable intangible assets 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
The number of businesses acquired by the entity during the period.
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| - Definition
The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.
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| - Definition
Number of shares of stock issued during the period pursuant to acquisitions.
+ References
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| - Definition
Value of stock issued pursuant to acquisitions during the period.
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