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Investment Securities - Narrative (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2009
Dec. 31, 2014
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]        
Carrying value of securities pledged as collateral $ 458,131frme_SecuritiesPledgedAsCollateral     $ 449,408frme_SecuritiesPledgedAsCollateral
Book value of securities sold under agreements to repurchase 129,110us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts     120,027us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts
Net realized gains on sales of available for sale securities 1,025us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet 581us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet    
Corporate obligations        
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]        
Net impairment losses recognized in earnings     500us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Net realized gains on sales of available for sale securities $ 45us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember