| **Pension and Other Post Retirement Benefit Plans - Narrative (Details) (USD $)In Thousands, unless otherwise specified** | 0 Months Ended | 12 Months Ended                                                                                                                              |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------------------------------------------------------------------------------------------------------------------------------------|
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| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Percentage of employees covered by plan                                                                                   |           | 25.00%frme_DefinedBenefitPlanPlanParticipantsEmployeesCoveredByPlanPercentage                                                                |   |   |
| Minimum participant attained age                                                                                          | 55 years       |                                                                                                                                         |   |   |
| Requisite service period                                                                                                  | 10 years       |                                                                                                                                         |   |   |
| Discount rate                                                                                                             |           | 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate                                                       | 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate | 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate |
| Percentage of the premiums paid by plan participants                                                                      |           | 100.00%frme_DefinedBenefitPlanOtherPostretirementBenefitPercentOfPremiumsPaidByParticipant                                                   |   |   |
| Accumulated benefit obligation                                                                                            |           | $ 80,650us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation                                                                               | $ 62,263us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation |   |
| Amount of discretionary contributions made by employer to defined contribution plan, percent                              |           | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent                                                                     |   |   |
| Maximum annual contribution percentage per employee                                                                       |           | 6.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent                                                             |   |   |
| Requisite service period                                                                                                  |           | 1000 hours                                                                                                                                   |   |   |
| Employer percentage contribution                                                                                          |           | 2.00%frme_DefinedContributionPlanEmployerContributionPercentofEmployeesGrossPay                                                              |   |   |
| Vesting period                                                                                                            |           | 5 years                                                                                                                                      |   |   |
| Expense recognized in the period                                                                                          |           | 3,396us-gaap_DefinedContributionPlanCostRecognized                                                                                           | 3,138us-gaap_DefinedContributionPlanCostRecognized | 2,914us-gaap_DefinedContributionPlanCostRecognized |
| Obligations payable                                                                                                       |           | 1,811us-gaap_OtherPostretirementDefinedBenefitPlanLiabilitiesCurrentAndNoncurrent                                                            | 1,534us-gaap_OtherPostretirementDefinedBenefitPlanLiabilitiesCurrentAndNoncurrent |   |
| Post retirement plan expense                                                                                              |           | 97us-gaap_OtherPostretirementBenefitExpense                                                                                                  | 519us-gaap_OtherPostretirementBenefitExpense | 477us-gaap_OtherPostretirementBenefitExpense |
| Fair value of plan assets                                                                                                 |           | 77,139us-gaap_DefinedBenefitPlanFairValueOfPlanAssets                                                                                        | 69,871us-gaap_DefinedBenefitPlanFairValueOfPlanAssets | 62,865us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
| Minimum                                                                                                                   |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Age of plan participants                                                                                                  |           | 55 years                                                                                                                                     |   |   |
| Maximum                                                                                                                   |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Age of plan participants                                                                                                  |           | 65 years                                                                                                                                     |   |   |
| Quoted Prices in Active Markets for Identical Assets (Level 1)                                                            |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Fair value of plan assets                                                                                                 |           | 57,413us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member   | 51,176us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member |   |
| Significant Other Observable Inputs (Level 2)                                                                             |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Fair value of plan assets                                                                                                 |           | $ 19,726us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member | $ 18,695us-gaap_DefinedBenefitPlanFairValueOfPlanAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |   |
| Debt securities | Minimum                                                                                                 |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Maturities of debt securities                                                                                             |           | 15 days                                                                                                                                      | 2 days |   |
| Debt securities | Maximum                                                                                                 |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Maturities of debt securities                                                                                             |           | 9 years 7 months 3 days                                                                                                                      | 9 years 7 months 10 days |   |
| Debt securities | Weighted Average                                                                                        |           |                                                                                                                                         |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                          |           |                                                                                                                                         |   |   |
| Maturities of debt securities                                                                                             |           | 5 years 0 months 7 days                                                                                                                      | 4 years 7 months 18 days |   |

| X                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Minimum Participant Attained Age
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Other Postretirement Benefit, Participant Attained Age
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Other Postretirement Benefit, Percent of Premiums Paid by Participant
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Plan Assets, Maturity Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Plan Participants, Employees Covered By Plan, Percentage
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefits Plan, Minimum Requisite Service Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Contribution Plan, Employer Contribution, Percent of Employees' Gross Pay
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Contribution Plan, Requisite Service Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Contribution Plan, Vesting Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
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| - Definition
For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The interest rate used to adjust for the time value of money for the plan.
+ References
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| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the cost recognized during the period for defined contribution plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of costs recognized during the period for non-pension postretirement benefits, such as medical, dental and life insurance.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This represents the entire liability recognized in the balance sheet that is associated with other postretirement defined benefit plans (excluding pension plans).
+ References
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                          |
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| X                                                                                                                                                                          |
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| X                                                                                                                               |
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| X                                                                                                                               |
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