| **Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                        | Dec. 31, 2013                                                                                                                                                                                        |
|-----------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | $ 529,212us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                             | $ 537,848us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                             |
| Gross Unrealized Gains                                                                                                                        | 20,581us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                      | 7,896us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                       |
| Gross Unrealized Losses                                                                                                                       | 250us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                         | 9,543us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                       |
| Investment securities available for sale                                                                                                      | 549,543us-gaap_AvailableForSaleSecurities                                                                                                                                                            | 536,201us-gaap_AvailableForSaleSecurities                                                                                                                                                            |
| Amortized Cost                                                                                                                                | 631,088us-gaap_HeldToMaturitySecurities                                                                                                                                                              | 559,378us-gaap_HeldToMaturitySecurities                                                                                                                                                              |
| Gross Unrealized Gains                                                                                                                        | 17,243us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain                                                                                                                             | 5,282us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain                                                                                                                              |
| Gross Unrealized Losses                                                                                                                       | 608us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss                                                                                                                                | 3,813us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss                                                                                                                              |
| Fair Value                                                                                                                                    | 647,723us-gaap_HeldToMaturitySecuritiesFairValue                                                                                                                                                     | 560,847us-gaap_HeldToMaturitySecuritiesFairValue                                                                                                                                                     |
| Amortized Cost                                                                                                                                | 1,160,300frme_SecuritiesAmortizedCost                                                                                                                                                                | 1,097,226frme_SecuritiesAmortizedCost                                                                                                                                                                |
| Gross Unrealized Gains                                                                                                                        | 37,824frme_SecuritiesGrossUnrealizedGains                                                                                                                                                            | 13,178frme_SecuritiesGrossUnrealizedGains                                                                                                                                                            |
| Gross Unrealized Losses                                                                                                                       | 858frme_SecuritiesGrossUnrealizedLosses                                                                                                                                                              | 13,356frme_SecuritiesGrossUnrealizedLosses                                                                                                                                                           |
| Fair Value                                                                                                                                    | 1,197,266frme_SecuritiesFairValue                                                                                                                                                                    | 1,097,048frme_SecuritiesFairValue                                                                                                                                                                    |
| U.S. Treasury                                                                                                                                 |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                |                                                                                                                                                                                                 | 15,914us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember                                                                      |
| Gross Unrealized Gains                                                                                                                        |                                                                                                                                                                                                 | 80us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember                                                |
| Gross Unrealized Losses                                                                                                                       |                                                                                                                                                                                                 | 21us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember                                                |
| Investment securities available for sale                                                                                                      |                                                                                                                                                                                                 | 15,973us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USTreasurySecuritiesMember                                                                                   |
| Government Agency and Municipal Bonds and Notes                                                                                               |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 100us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                           | 3,550us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                         |
| Gross Unrealized Gains                                                                                                                        | 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                   | 12us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                  |
| Gross Unrealized Losses                                                                                                                       |                                                                                                                                                                                            | 17us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                  |
| Investment securities available for sale                                                                                                      | 109us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                        | 3,545us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                      |
| State and municipal                                                                                                                           |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 216,915us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                         | 231,005us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                         |
| Gross Unrealized Gains                                                                                                                        | 11,801us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                | 3,878us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 |
| Gross Unrealized Losses                                                                                                                       | 123us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                   | 3,896us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 |
| Investment securities available for sale                                                                                                      | 228,593us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                      | 230,987us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                      |
| Amortized Cost                                                                                                                                | 204,443us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                        | 145,941us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                        |
| Gross Unrealized Gains                                                                                                                        | 5,716us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                        | 62us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                           |
| Gross Unrealized Losses                                                                                                                       | 96us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                           | 91us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                           |
| Fair Value                                                                                                                                    | 210,063us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                               | 145,912us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                               |
| U.S. Government-sponsored mortgage-backed securities                                                                                          |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 310,460us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                         | 279,299us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                         |
| Gross Unrealized Gains                                                                                                                        | 8,771us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 3,926us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| Gross Unrealized Losses                                                                                                                       | 127us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   | 1,973us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| Investment securities available for sale                                                                                                      | 319,104us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                      | 281,252us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                      |
| Amortized Cost                                                                                                                                | 426,645us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                        | 413,437us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                        |
| Gross Unrealized Gains                                                                                                                        | 11,527us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember       | 5,220us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember        |
| Gross Unrealized Losses                                                                                                                       | 512us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember          | 3,722us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember        |
| Fair Value                                                                                                                                    | 437,660us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                               | 414,935us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                               |
| Corporate obligations                                                                                                                         |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 31us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                       | 6,374us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                    |
| Gross Unrealized Losses                                                                                                                       |                                                                                                                                                                                            | 3,636us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                          |
| Investment securities available for sale                                                                                                      | 31us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                    | 2,738us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                 |
| Equity securities                                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 1,706us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                           | 1,706us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                           |
| Investment securities available for sale                                                                                                      | $ 1,706us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                      | $ 1,706us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                      |

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| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
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| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
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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.
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| - Definition
Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.
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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.
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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.
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| - Definition
Fair value of investments in debt securities classified as held-to-maturity.
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