| **Business Combinations - Narrative (Details) (USD $)Share data in Millions, except Per Share data, unless otherwise specified** | 0 Months Ended                                                                                                                                                                                                                          | 12 Months Ended                                                                                                                                                                   | 0 Months Ended                                                                                                                                                                   |                                                                                                                                                                                                                                                                                    |
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| **Business Acquisition [Line Items]**                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| Assets                                                                                                                           |                                                                                                                                                                                                                                    | $ 5,824,127,000us-gaap_Assets                                                                                                                                                     |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               | $ 5,877,521,000us-gaap_Assets |
| 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                                                                                                                     |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| 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)                                                                                | 1.6us-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                                                                                               |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| 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 operating revenue                                                                                                      |                                                                                                                                                                                                                                    | 1,600,000us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual/ 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]**                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| Pro forma operating revenue                                                                                                      |                                                                                                                                                                                                                                    |                                                                                                                                                                              | 4,900,000us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |                                                                                                                                                                                                                                                                               |   |
| Pro forma nonrecurring expense                                                                                                   |                                                                                                                                                                                                                                    |                                                                                                                                                                              | 9,500,000frme_BusinessAcquisitionProFormaNonrecurringExpense/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                       |                                                                                                                                                                                                                                                                               |   |
| Merger with C Financial Corporation | Subsequent Event                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| **Business Acquisition [Line Items]**                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| Total purchase price                                                                                                             |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             | 14,500,000us-gaap_BusinessCombinationConsiderationTransferred1/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFinancialCorporationMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                        |   |
| Assets                                                                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             | 6,000,000,000us-gaap_Assets/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFinancialCorporationMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                                                                           |   |
| Merger with C Financial Corporation | First Merchants Corporation | Subsequent Event                                             |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| **Business Acquisition [Line Items]**                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| Assets                                                                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             | 5,800,000,000us-gaap_Assets/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFinancialCorporationMember/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                         |   |
| Merger with C Financial Corporation | C Financial Corporation | Subsequent Event                                                 |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| **Business Acquisition [Line Items]**                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             |                                                                                                                                                                                                                                                                               |   |
| Cash received by shareholders for each share of common stock held                                                                |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             | $ 6.738frme_BusinessCombinationsAcquireeCashReceivedForEachShareCurrentlyOwned/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFinancialCorporationMember/ dei_LegalEntityAxis= frme_CFinancialCorporationMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember |   |
| Assets                                                                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                              |                                                                                                                                                                             | $ 138,000,000us-gaap_Assets/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFinancialCorporationMember/ dei_LegalEntityAxis= frme_CFinancialCorporationMember/ us-gaap_SubsequentEventTypeAxis= us-gaap_SubsequentEventMember                                                    |   |

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Business Acquisition, Pro Forma Nonrecurring Expense
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| - Definition
Business Combinations, Acquiree, Cash Received For Each Share Currently Owned
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| - Definition
Business Combinations, Acquiree, Shares of Stock Received For Each Share Currently Owned
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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
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ 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.
+ References
No definition available.
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| - Definition
Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.
+ References
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| - Definition
This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.
+ References
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| - Definition
The amount of identifiable intangible assets recognized as of the acquisition date.
+ References
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| - Definition
Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.
+ References
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| - Definition
The number of businesses acquired by the entity during the period.
+ References
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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.
+ References
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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.
+ References
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