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Derivative Instruments and Other Hedging Activities - Effect of Hedging Instruments on Consolidated Financial Statements (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in OCI net of taxes (Effective Portion) $ 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet  
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)   (392)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet  
Amount of Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing) (1)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet 1us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet  
Amount of Gain (Loss) Recognized in Income on Derivatives (1)us-gaap_DerivativeGainLossOnDerivativeNet 209us-gaap_DerivativeGainLossOnDerivativeNet (1)us-gaap_DerivativeGainLossOnDerivativeNet
Not Designated as Hedging Instruments [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives (6,451)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 (1,757)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		  
Not Designated as Hedging Instruments [Member] | Interest Rate Contracts [Member] | Other Income (Expense) [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives 2,513us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherIncomeMember		 2,991us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherIncomeMember		  
Not Designated as Hedging Instruments [Member] | Forward Sale Contracts [Member] | Mortgage Income [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives (3,225)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ibkc_ForwardSaleContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= ibkc_MortgageIncomeMember		 (1,716)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ibkc_ForwardSaleContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= ibkc_MortgageIncomeMember		  
Not Designated as Hedging Instruments [Member] | Written and Purchased Options [Member] | Mortgage Income [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives (5,739)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ibkc_WrittenAndPurchaseOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= ibkc_MortgageIncomeMember		 (3,032)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ibkc_WrittenAndPurchaseOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= ibkc_MortgageIncomeMember		  
Cash Flow Hedging [Member] | Interest Rate Contracts [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Amount of Gain (Loss) Recognized in OCI net of taxes (Effective Portion) 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		  
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)   (392)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		  
Amount of Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing) $ (1)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 $ 1us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember