| **Investment Securities - Narrative (Detail) (USD $)**                                     | 12 Months Ended                                                                                                                                                                                                           |
|--------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Fair value                                                                                 | $ 72,390,000frme_SecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                      | $ 477,132,000frme_SecuritiesContinuousUnrealizedLossPositionFairValue |   |   |
| Gross Unrealized Losses                                                                    | 250,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                                          | 9,543,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |   |   |
| Gross Unrealized Losses                                                                    | 608,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss                                                                                                                                                 | 3,813,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss |   |   |
| Amortized Cost                                                                             | 529,212,000us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                                                | 537,848,000us-gaap_AvailableForSaleSecuritiesAmortizedCost |   |   |
| Carrying value of securities pledged as collateral                                         | 449,408,000frme_SecuritiesPledgedAsCollateral                                                                                                                                                                             | 373,533,000frme_SecuritiesPledgedAsCollateral | 345,866,000frme_SecuritiesPledgedAsCollateral |   |
| Gross gains on securities sold                                                             | 3,581,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains                                                                                                                                                             | 487,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 2,389,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |   |
| Gross losses                                                                               | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses                                                                                                                                                                    | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |   |
| Gross Unrealized Losses                                                                    | 250,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                  | 9,543,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss |   |   |
| Corporate obligations                                                                      |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Amortized Cost                                                                             |                                                                                                                                                                                                                      | 6,400,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |   |   |
| Gross gains on securities sold                                                             | 1,900,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                     |   |   |   |
| Gross Unrealized Losses                                                                    |                                                                                                                                                                                                                      | 3,636,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |   |   |
| Accumulated OTTI on securities                                                             | 9,400,000frme_SecuritiesWithOtherThanTemporaryImpairmentAccumulatedOtherThanTemporaryImpairment/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                   |   |   |   |
| Net impairment losses recognized in earnings                                               |                                                                                                                                                                                                                      |   |   | 500,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |
| Investments in Debt and Equity Securities, Reported at Less than Historical Cost           |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Historical cost                                                                            | 73,249,000frme_InvestmentHistoricalCost/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember                                                            | 490,488,000frme_InvestmentHistoricalCost/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember |   |   |
| Fair value                                                                                 | 72,390,000frme_SecuritiesContinuousUnrealizedLossPositionFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember                                 | 477,132,000frme_SecuritiesContinuousUnrealizedLossPositionFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember |   |   |
| Percent of the Corporation's available for sale and held to maturity portfolio             | 6.10%frme_PercentageOfTotalInvestmentPortfolioFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember                                            | 43.60%frme_PercentageOfTotalInvestmentPortfolioFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember |   |   |
| U.S. Treasury Securities                                                                   |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Gross Unrealized Losses                                                                    |                                                                                                                                                                                                                      | 21,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember |   |   |
| Amortized Cost                                                                             |                                                                                                                                                                                                                      | 15,914,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember |   |   |
| U.S. Government-sponsored agency securities                                                |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Gross Unrealized Losses                                                                    |                                                                                                                                                                                                                 | 17,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember |   |   |
| Amortized Cost                                                                             | 100,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                            | 3,550,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember |   |   |
| U.S. Government-sponsored mortgage-backed securities                                       |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Gross Unrealized Losses                                                                    | 127,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                    | 1,973,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |   |   |
| Available-for-sale, number of securities in unrealized loss positions                      | 4us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |   |   |   |
| Gross Unrealized Losses                                                                    | 512,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                           | 3,722,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |   |   |
| Held-to-maturity, number of securities in unrealized loss positions                        | 13us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   |   |   |   |
| Amortized Cost                                                                             | 310,460,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                          | 279,299,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |   |   |
| State and municipal                                                                        |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Gross Unrealized Losses                                                                    | 123,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                    | 3,896,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |   |   |
| Available-for-sale, number of securities in unrealized loss positions                      | 22us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                |   |   |   |
| Gross Unrealized Losses                                                                    | 96,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                            | 91,000us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |   |   |
| Held-to-maturity, number of securities in unrealized loss positions                        | 16us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                   |   |   |   |
| Amortized Cost                                                                             | 216,915,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                          | 231,005,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |   |   |
| Corporate obligations                                                                      |                                                                                                                                                                                                                      |   |   |   |
| **Schedule of Available for sale Securities and Held to maturity Securities [Line Items]** |                                                                                                                                                                                                                      |   |   |   |
| Gross Unrealized Losses                                                                    |                                                                                                                                                                                                                 | 3,636,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember |   |   |
| Amortized Cost                                                                             | $ 31,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                      | $ 6,374,000us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember |   |   |

| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Investment Historical Cost
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Percentage of Total Investment Portfolio, Fair Value
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Securities, Continuous Unrealized Loss Position, Fair Value
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| - Definition
Securities Pledged as Collateral
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| - Definition
Securities With Other-Than-Temporary Impairment, Accumulated Other-Than-Temporary Impairment
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
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| - Definition
This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
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| - Definition
Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.
+ References
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| - Definition
Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.
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| - Definition
Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.
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| - Definition
The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.
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