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Securities (Details Textual) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]        
Gain on sale of investments $ 4us-gaap_GainLossOnSaleOfSecuritiesNet $ 0us-gaap_GainLossOnSaleOfSecuritiesNet    
Gross unrealized losses 3,688us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax   8,189us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax  
Cumulative OTTI charges 13,995us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 14,079us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 14,050us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 14,079us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Amortized Cost 882,463us-gaap_AvailableForSaleSecuritiesAmortizedCost   880,946us-gaap_AvailableForSaleSecuritiesAmortizedCost  
Securities available-for-sale $ 906,341us-gaap_AvailableForSaleSecurities   $ 897,053us-gaap_AvailableForSaleSecurities  
Minimum [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Basis point spread on variable rate 1.60%thff_AvailableforSaleSecuritiesBasisPointSpreadonVariableRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Minimum [Member] | Standard Poors [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Credit Quality Indicator Pricing 4.4thff_CreditQualityIndicatorPricing
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= thff_StandardPoorsMember
     
Minimum [Member] | Moody Investor Service [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Credit Quality Indicator Pricing 0.32thff_CreditQualityIndicatorPricing
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= thff_MoodyInvestorServiceMember
     
Maximum [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Basis point spread on variable rate 1.80%thff_AvailableforSaleSecuritiesBasisPointSpreadonVariableRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Maximum [Member] | Standard Poors [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Credit Quality Indicator Pricing 90.3thff_CreditQualityIndicatorPricing
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= thff_StandardPoorsMember
     
Maximum [Member] | Moody Investor Service [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Credit Quality Indicator Pricing 90.5thff_CreditQualityIndicatorPricing
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= thff_MoodyInvestorServiceMember