| **Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                           | Dec. 31, 2014                                                                                                                                                                           |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Schedule of Investments [Line Items]**                                                                                                                           |                                                                                                                                                                                    |                                                                                                                                                                                    |
| Fair value                                                                                                                                                         | $ 55,004frme_SecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                        | $ 72,390frme_SecuritiesContinuousUnrealizedLossPositionFairValue                                                                                                                        |
| Investments reported at less than historical cost:                                                                                                                 |                                                                                                                                                                                    |                                                                                                                                                                                    |
| **Schedule of Investments [Line Items]**                                                                                                                           |                                                                                                                                                                                    |                                                                                                                                                                                    |
| Historical cost                                                                                                                                                    | 55,599frme_InvestmentHistoricalCost/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember                              | 73,249frme_InvestmentHistoricalCost/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember                              |
| Fair value                                                                                                                                                         | $ 55,004frme_SecuritiesContinuousUnrealizedLossPositionFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember | $ 72,390frme_SecuritiesContinuousUnrealizedLossPositionFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember |
| Percent of the Corporation's available for sale and held to maturity portfolio                                                                                     | 4.60%frme_PercentageOfTotalInvestmentPortfolioFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember          | 6.10%frme_PercentageOfTotalInvestmentPortfolioFairValue/ invest_InvestmentSecuritiesClassAxis= frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember          |

| X                                                                                                                                                                                                                                        |
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| - Definition
Investment Historical Cost
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Total Investment Portfolio, Fair Value
+ References
No definition available.
+ Details**Name:**frme_PercentageOfTotalInvestmentPortfolioFairValue**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**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                                                                                                                                                                  |
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| X                                                                                                                                                                                                            |
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| - Details**Name:**invest_InvestmentSecuritiesClassAxis=frme_InvestmentsInDebtAndEquitySecuritiesReportedAtLessThanHistoricalCostMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |