| **Stockholders' Equity (Details) (USD $)**                                                                                           | 12 Months Ended                                                                                                          | 0 Months Ended                                              |                                                       |
|--------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------|-------------------------------------------------------|
||
| **Class of Stock [Line Items]**                                                                                                      |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| Stockholders' equity                                                                                                                 | $ 726,827,000us-gaap_StockholdersEquity                                                                                  | $ 634,923,000us-gaap_StockholdersEquity                     | $ 552,236,000us-gaap_StockholdersEquity               |   |   |   |   |   |   | $ 514,467,000us-gaap_StockholdersEquity |
| Dividend reinvestment and stock purchase plan, stockholder's maximum amount of cash payments per quarter in purchase of common stock | 2,500frme_DividendReinvestmentAndStockPurchasePlanStockholderMaximumAmountOfCashPaymentsPerQuarterPurchasesOfCommonStock |                                                        |                                                  |   |   |   |   |   |   |   |
| Preferred stock, liquidation value (in dollars per share)                                                                            | $ 1,000us-gaap_PreferredStockLiquidationPreference                                                                       | $ 1,000us-gaap_PreferredStockLiquidationPreference          |                                                  |   |   |   |   |   |   |   |
| Price of preferred stock redeemed                                                                                                    | 1,067,000us-gaap_StockRedeemedOrCalledDuringPeriodValue                                                                  | 491,000us-gaap_StockRedeemedOrCalledDuringPeriodValue       | 235,000us-gaap_StockRedeemedOrCalledDuringPeriodValue |   |   |   |   |   |   |   |
| Preferred stock, shares outstanding                                                                                                  | 125us-gaap_PreferredStockSharesOutstanding                                                                               | 125us-gaap_PreferredStockSharesOutstanding                  |                                                  |   |   |   |   |   |   |   |
| Value of stock issued as a part of acquisition                                                                                       | 34,981,000us-gaap_StockIssuedDuringPeriodValueAcquisitions                                                               | 135,642,000us-gaap_StockIssuedDuringPeriodValueAcquisitions |                                                  |   |   |   |   |   |   |   |
| Merger with CFS Bancorp, Inc. (Citizens)                                                                                             |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| **Class of Stock [Line Items]**                                                                                                      |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| Percentage of interest acquired                                                                                                      |                                                                                                                     |                                                        |                                                  | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired/ 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 |   |   |   |   |   |   |
| Merger with Community Bancshares, Inc.                                                                                               |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| **Class of Stock [Line Items]**                                                                                                      |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| Percentage of interest acquired                                                                                                      |                                                                                                                     |                                                        |                                                  |   | 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired/ 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 |   |   |   |   |   |
| 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 |   |   |   |   |   |
| Total purchase price                                                                                                                 |                                                                                                                     |                                                        |                                                  |   | 49,189,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |   |   |   |   |   |
| Series B Preferred Stock                                                                                                             |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| **Class of Stock [Line Items]**                                                                                                      |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| Number of new stock issued during the period (shares)                                                                                |                                                                                                                     |                                                        |                                                  |   |   |   |   |   | 90,782.94us-gaap_StockIssuedDuringPeriodSharesNewIssues/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |
| Preferred stock, liquidation value (in dollars per share)                                                                            |                                                                                                                     |                                                        |                                                  |   |   |   |   |   | $ 1,000us-gaap_PreferredStockLiquidationPreference/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |
| Number of new stock issued during the period value                                                                                   |                                                                                                                     |                                                        |                                                  |   |   |   |   |   | 90,782,940us-gaap_StockIssuedDuringPeriodValueNewIssues/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |
| Preferred stock redeemed (in shares)                                                                                                 |                                                                                                                     |                                                        |                                                  |   |   | 34,043us-gaap_StockRedeemedOrCalledDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 34,044us-gaap_StockRedeemedOrCalledDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 22,695.94us-gaap_StockRedeemedOrCalledDuringPeriodShares/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |   |
| Price of preferred stock redeemed                                                                                                    |                                                                                                                     |                                                        |                                                  |   |   | 34,043,000us-gaap_StockRedeemedOrCalledDuringPeriodValue/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 34,044,000us-gaap_StockRedeemedOrCalledDuringPeriodValue/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 22,695,940us-gaap_StockRedeemedOrCalledDuringPeriodValue/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |   |
| Preferred stock, shares outstanding                                                                                                  | 0us-gaap_PreferredStockSharesOutstanding/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember         |                                                        |                                                  |   |   |   | 34,043us-gaap_PreferredStockSharesOutstanding/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 68,087us-gaap_PreferredStockSharesOutstanding/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |   |
| Series B Preferred Stock remaining liquidation amount                                                                                |                                                                                                                     |                                                        |                                                  |   |   |   | 34,000,000frme_PreferredStockOutstandingAggregateLiquidationAmount/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember | 68,000,000frme_PreferredStockOutstandingAggregateLiquidationAmount/ us-gaap_StatementClassOfStockAxis= us-gaap_SeriesBPreferredStockMember |   |   |
| Subsidiaries                                                                                                                         |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| **Class of Stock [Line Items]**                                                                                                      |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| Amount available for dividends                                                                                                       | 23,340,000us-gaap_RetainedEarningsUnappropriated/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                        |                                                        |                                                  |   |   |   |   |   |   |   |
| Stockholders' equity                                                                                                                 | 818,420,000us-gaap_StockholdersEquity/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                                   |                                                        |                                                  |   |   |   |   |   |   |   |
| Stockholders' equity restricted from dividend distribution                                                                           | $ 795,080,000us-gaap_RetainedEarningsAppropriated/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                       |                                                        |                                                  |   |   |   |   |   |   |   |
| CFS Bancorp | Merger with CFS Bancorp, Inc. (Citizens)                                                                               |                                                                                                                     |                                                        |                                                  |   |   |   |   |   |   |   |
| **Class of Stock [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
Dividend Reinvestment And Stock Purchase Plan, Stockholder, Maximum Amount Of Cash Payments Allowed Per Quarter, Purchases Of Common Stock
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Preferred Stock, Outstanding Aggregate Liquidation Amount
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| - Definition
Percentage of voting equity interests acquired at the acquisition date in the business combination.
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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
Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.
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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
The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A segregation of retained earnings which is unavailable for dividend distribution. Includes also retained earnings appropriated for loss contingencies.
+ References
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| - Definition
A segregation of retained earnings which is available for dividend distribution. Earnings not paid out as dividends but instead reinvested in the core business or used to pay off debt. Unappropriated profit is part of shareholder equity. Also called cumulative distributions or earned surplus or accumulated earnings or unappropriated profit.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares of stock issued during the period pursuant to acquisitions.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of new stock issued during the period.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of stock issued pursuant to acquisitions during the period.
+ References
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| - Definition
Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5
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| - Definition
Number of stock bought back by the entity at the exercise price or redemption price.
+ References
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| - Definition
Equity impact of the value of stock bought back by the entity at the exercise price or redemption price.
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