| **Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                                         | Dec. 31, 2013                                                                                                                                                                                                                           |
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| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | $ 3,442us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                | $ 237,965us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                |
| Less than 12 Months, Unrealized Losses                                                                                                                                                                                                | 20us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss                                                                                                                                   | 5,579us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss                                                                                                                                  |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 15,297us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                 | 5,977us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                    |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                | 230us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss                                                                                                                                  | 3,964us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss                                                                                                                                  |
| Total, Fair Value                                                                                                                                                                                                                     | 18,739us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                     | 243,942us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                      |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 250us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                  | 9,543us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                  |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 14,910us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                   | 230,366us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                    |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                                          | 178us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss                                                                                                                                    | 3,553us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss                                                                                                                                    |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 38,741us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                   | 2,824us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                      |
| 12 Months or Longer, Gross Unrealized Losses                                                                                                                                                                                          | 430us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss                                                                                                                                    | 260us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss                                                                                                                                      |
| Total, Fair Value                                                                                                                                                                                                                     | 53,651us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                       | 233,190us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                        |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 608us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                    | 3,813us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss                                                                                                                                                    |
| **Available-for-sale Securities and Held-to-maturity Securities [Abstract]**                                                                                                                                                          |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 18,352frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                                    | 468,331frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue                                                                                                                                                     |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                                          | 198frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses                                                                                                                                                 | 9,132frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses                                                                                                                                                 |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 54,038frme_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                                    | 8,801frme_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue                                                                                                                                                       |
| 12 Months or Longer, Gross Unrealized Losses                                                                                                                                                                                          | 660frme_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses                                                                                                                                                     | 4,224frme_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses                                                                                                                                                     |
| Total, Fair Value                                                                                                                                                                                                                     | 72,390frme_SecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                                        | 477,132frme_SecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                                                                         |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 858frme_SecuritiesContinuousUnrealizedLossPositionAggregateLosses                                                                                                                                                                     | 13,356frme_SecuritiesContinuousUnrealizedLossPositionAggregateLosses                                                                                                                                                                    |
| State and municipal                                                                                                                                                                                                                   |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 5,119us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                          | 17,318us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                           |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                                          | 96us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                           | 91us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                             |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 250us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                            | 184us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                              |
| Total, Fair Value                                                                                                                                                                                                                     | 5,369us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                              | 17,502us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                               |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 96us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                           | 91us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                             |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                                                  |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 9,791us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                          | 213,048us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                          |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                                          | 82us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                           | 3,462us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                          |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 38,491us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                         | 2,640us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                            |
| 12 Months or Longer, Gross Unrealized Losses                                                                                                                                                                                          | 430us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                          | 260us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                            |
| Total, Fair Value                                                                                                                                                                                                                     | 48,282us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                             | 215,688us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                              |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 512us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                          | 3,722us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_DebtSecurityAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                          |
| U.S. Treasury                                                                                                                                                                                                                         |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       |                                                                                                                                                                                                                                  | 4,875us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                               |
| Less than 12 Months, Unrealized Losses                                                                                                                                                                                                |                                                                                                                                                                                                                                  | 21us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                                |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                               |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                               |
| Total, Fair Value                                                                                                                                                                                                                     |                                                                                                                                                                                                                                  | 4,875us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                                                   |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        |                                                                                                                                                                                                                                  | 21us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasurySecuritiesMember                                                                |
| Government Agency and Municipal Bonds and Notes                                                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       |                                                                                                                                                                                                                                  | 3,433us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                 |
| Less than 12 Months, Unrealized Losses                                                                                                                                                                                                |                                                                                                                                                                                                                                  | 17us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                  |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                               |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                               |
| Total, Fair Value                                                                                                                                                                                                                     |                                                                                                                                                                                                                                  | 3,433us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                     |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        |                                                                                                                                                                                                                                  | 17us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                  |
| State and municipal                                                                                                                                                                                                                   |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 1,256us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 | 111,791us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 |
| Less than 12 Months, Unrealized Losses                                                                                                                                                                                                | 7us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                   | 3,840us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 9,850us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 | 583us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                     |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                | 116us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                 | 56us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                    |
| Total, Fair Value                                                                                                                                                                                                                     | 11,106us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                    | 112,374us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                     |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 123us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                 | 3,896us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                 |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                                                  |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| Less than 12 Months, Fair Value                                                                                                                                                                                                       | 2,186us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 117,866us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| Less than 12 Months, Unrealized Losses                                                                                                                                                                                                | 13us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember  | 1,701us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       | 5,447us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 2,683us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                | 114us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 272us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   |
| Total, Fair Value                                                                                                                                                                                                                     | 7,633us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                     | 120,549us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                     |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        | 127us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                 | 1,973us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                 |
| Corporate obligations                                                                                                                                                                                                                 |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| **Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]**                                                                                                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                    |
| 12 Months or Longer, Fair Value                                                                                                                                                                                                       |                                                                                                                                                                                                                                  | 2,711us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                            |
| 12 Months or Longer, Unrealized Losses                                                                                                                                                                                                |                                                                                                                                                                                                                                  | 3,636us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                          |
| Total, Fair Value                                                                                                                                                                                                                     |                                                                                                                                                                                                                                  | 2,711us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                |
| Total, Gross Unrealized Losses                                                                                                                                                                                                        |                                                                                                                                                                                                                                  | $ 3,636us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                        |

| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Aggregate Losses
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPositionAggregateLosses**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Fair Value
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPositionFairValue**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Aggregate Losses
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value
+ References
No definition available.
+ Details**Name:**frme_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.
+ References
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