| **Borrowings - Schedule of Maturities of Long Term Debt (Details) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| Securities Sold Under Repurchase Agreements                                                                         |                                                                                                                                                                  |   |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                  |   |
| 2015                                                                                                                | $ 124,539us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |   |
| 2016                                                                                                                |                                                                                                                                                             |   |
| Total                                                                                                               | 124,539us-gaap_LongTermDebt/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember                                                    |   |
| Maximum Amount Outstanding During Period                                                                            | 155,941frme_LongTermDebtMaximumAmountOutstandingDuringPeriod/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember                   | 161,814frme_LongTermDebtMaximumAmountOutstandingDuringPeriod/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
| Average Outstanding Amount                                                                                          | 130,910frme_LongTermDebtAverageOutstandingAmount/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember                               | 140,126frme_LongTermDebtAverageOutstandingAmount/ us-gaap_LongtermDebtTypeAxis= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
| Federal Home Loan Bank Advances                                                                                     |                                                                                                                                                                  |   |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                  |   |
| 2015                                                                                                                | 30,780us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                  |   |
| 2016                                                                                                                | 41,225us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                           |   |
| 2017                                                                                                                | 15,018us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                         |   |
| 2018                                                                                                                | 25,637us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                          |   |
| 2019                                                                                                                | 12,503us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                          |   |
| After 2019                                                                                                          | 20,101us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                       |   |
| Total                                                                                                               | 145,264us-gaap_LongTermDebt/ us-gaap_LongtermDebtTypeAxis= us-gaap_FederalHomeLoanBankAdvancesMember                                                                  |   |
| Subordinated Debentures and Term Loans                                                                              |                                                                                                                                                                  |   |
| **Debt Instrument [Line Items]**                                                                                    |                                                                                                                                                                  |   |
| 2015                                                                                                                | 108us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths/ us-gaap_LongtermDebtTypeAxis= frme_SubordinatedDebenturesAndTermLoansMember                 |   |
| 2017                                                                                                                |                                                                                                                                                             |   |
| After 2019                                                                                                          | 126,702us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive/ us-gaap_LongtermDebtTypeAxis= frme_SubordinatedDebenturesAndTermLoansMember                  |   |
| Total                                                                                                               | $ 126,810us-gaap_LongTermDebt/ us-gaap_LongtermDebtTypeAxis= frme_SubordinatedDebenturesAndTermLoansMember                                                            | $ 126,807us-gaap_LongTermDebt/ us-gaap_LongtermDebtTypeAxis= frme_SubordinatedDebenturesAndTermLoansMember |

| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Average Outstanding Amount
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Maximum Amount Outstanding During Period
+ References
No definition available.
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.
+ References
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| X                                                                                                                                                              |
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| X                                                                                                                                                                            |
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