| **Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                        | Dec. 31, 2014                                                                                                                                                                                        |
|-----------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | $ 517,276us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                             | $ 529,212us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                             |
| Gross Unrealized Gains                                                                                                                        | 21,151us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                      | 20,581us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                      |
| Gross Unrealized Losses                                                                                                                       | 372us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                         | 250us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                         |
| Investment securities available for sale                                                                                                      | 538,055us-gaap_AvailableForSaleSecurities                                                                                                                                                            | 549,543us-gaap_AvailableForSaleSecurities                                                                                                                                                            |
| **Held to maturity securities**                                                                                                               |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 651,418us-gaap_HeldToMaturitySecurities                                                                                                                                                              | 631,088us-gaap_HeldToMaturitySecurities                                                                                                                                                              |
| Gross Unrealized Gains                                                                                                                        | 19,417us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain                                                                                                                             | 17,243us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain                                                                                                                             |
| Gross Unrealized Losses                                                                                                                       | 224us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss                                                                                                                                | 608us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss                                                                                                                                |
| Investment securities held to maturity - fair value                                                                                           | 670,611us-gaap_HeldToMaturitySecuritiesFairValue                                                                                                                                                     | 647,723us-gaap_HeldToMaturitySecuritiesFairValue                                                                                                                                                     |
| Amortized Cost                                                                                                                                | 1,168,694frme_SecuritiesAmortizedCost                                                                                                                                                                | 1,160,300frme_SecuritiesAmortizedCost                                                                                                                                                                |
| Gross Unrealized Gains                                                                                                                        | 40,568frme_SecuritiesGrossUnrealizedGains                                                                                                                                                            | 37,824frme_SecuritiesGrossUnrealizedGains                                                                                                                                                            |
| Gross Unrealized Losses                                                                                                                       | 596frme_SecuritiesGrossUnrealizedLosses                                                                                                                                                              | 858frme_SecuritiesGrossUnrealizedLosses                                                                                                                                                              |
| Fair Value                                                                                                                                    | 1,208,666frme_SecuritiesFairValue                                                                                                                                                                    | 1,197,266frme_SecuritiesFairValue                                                                                                                                                                    |
| U.S. Government-sponsored agency securities                                                                                                   |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 100us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                           | 100us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                           |
| Gross Unrealized Gains                                                                                                                        | 7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                   | 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                   |
| Gross Unrealized Losses                                                                                                                       |                                                                                                                                                                                                 |                                                                                                                                                                                            |
| Investment securities available for sale                                                                                                      | 107us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                        | 109us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                        |
| State and municipal                                                                                                                           |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 249,699us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                         | 216,915us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                         |
| Gross Unrealized Gains                                                                                                                        | 12,195us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                | 11,801us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                |
| Gross Unrealized Losses                                                                                                                       | 310us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                   | 123us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                   |
| Investment securities available for sale                                                                                                      | 261,584us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                      | 228,593us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                      |
| **Held to maturity securities**                                                                                                               |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 225,225us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                        | 204,443us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                        |
| Gross Unrealized Gains                                                                                                                        | 6,429us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                        | 5,716us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                        |
| Gross Unrealized Losses                                                                                                                       | 76us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                           | 96us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                           |
| Investment securities held to maturity - fair value                                                                                           | 231,578us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                               | 210,063us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                               |
| U.S. Government-sponsored mortgage-backed securities                                                                                          |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 265,740us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                         | 310,460us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                         |
| Gross Unrealized Gains                                                                                                                        | 8,949us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 8,771us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| Gross Unrealized Losses                                                                                                                       | 62us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember    | 127us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   |
| Investment securities available for sale                                                                                                      | 274,627us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                      | 319,104us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                      |
| **Held to maturity securities**                                                                                                               |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 426,193us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                        | 426,645us-gaap_HeldToMaturitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                        |
| Gross Unrealized Gains                                                                                                                        | 12,988us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember       | 11,527us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember       |
| Gross Unrealized Losses                                                                                                                       | 148us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember          | 512us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember          |
| Investment securities held to maturity - fair value                                                                                           | 439,033us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                               | 437,660us-gaap_HeldToMaturitySecuritiesFairValue/ invest_InvestmentSecuritiesClassAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                               |
| Corporate obligations                                                                                                                         |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| Amortized Cost                                                                                                                                | 31us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                       | 31us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                       |
| Gross Unrealized Gains                                                                                                                        |                                                                                                                                                                                            |                                                                                                                                                                                                 |
| Gross Unrealized Losses                                                                                                                       |                                                                                                                                                                                            |                                                                                                                                                                                            |
| Investment securities available for sale                                                                                                      | 31us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                    | 31us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                    |
| Equity securities                                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| **Available for sale securities**                                                                                                             |                                                                                                                                                                                                 |                                                                                                                                                                                                 |
| 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.
+ References
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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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| - 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.
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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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| - Definition
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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