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Loans - Schedule of Modified TDRs (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Receivables [Abstract]    
Extended maturities $ 0ibkc_FinancingReceivableModificationsExtendedMaturities $ 0ibkc_FinancingReceivableModificationsExtendedMaturities
Interest rate adjustment 0ibkc_FinancingReceivableModificationsInterestRateAdjustment 0ibkc_FinancingReceivableModificationsInterestRateAdjustment
Maturity and interest rate adjustment 0ibkc_FinancingReceivableModificationsMaturityAndInterestRateAdjustment 0ibkc_FinancingReceivableModificationsMaturityAndInterestRateAdjustment
Movement to or extension of interest-rate only payments 0ibkc_FinancingReceivableModificationsExtensionOfInterestRateOnlyPayments 0ibkc_FinancingReceivableModificationsExtensionOfInterestRateOnlyPayments
Forbearance   12,975ibkc_FinancingReceivableModificationsForbearance
Other concession(s)   1,587ibkc_FinancingReceivableModificationsOtherConcessions
Total   $ 14,562ibkc_NonCoveredTroubledDebtRestructuringsModifiedDuringCurrentPeriod