XML 78 R103.htm IDEA: XBRL DOCUMENT v2.4.1.9
Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Loss Contingencies [Line Items]        
Reserve for unfunded lending commitments $ 12,849ibkc_ReserveForUnfundedCommitments $ 11,801ibkc_ReserveForUnfundedCommitments $ 11,519ibkc_ReserveForUnfundedCommitments $ 11,147ibkc_ReserveForUnfundedCommitments
Credit Risk Contract [Member]        
Loss Contingencies [Line Items]        
Commitments to grant loans 148,151ibkc_CommitmentsToGrantLoans
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 161,350ibkc_CommitmentsToGrantLoans
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		    
Unfunded commitments under lines of credit 4,095,321ibkc_LineOfCreditFacilityUnfundedCommitments
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 4,007,954ibkc_LineOfCreditFacilityUnfundedCommitments
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		    
Commercial and standby letters of credit 133,247us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 134,882us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		    
Reserve for unfunded lending commitments $ 12,849ibkc_ReserveForUnfundedCommitments
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 $ 11,801ibkc_ReserveForUnfundedCommitments
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember