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Transfers and Servicing of Financial Assets (Including Mortgage Banking Activity) - Changes in Carrying Amount of Mortgage Loans Held-for-sale (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mortgage Banking [Abstract]      
Balance at beginning of period $ 128,442us-gaap_LoansHeldForSaleMortgages $ 267,475us-gaap_LoansHeldForSaleMortgages $ 153,013us-gaap_LoansHeldForSaleMortgages
Originations and Purchases 1,675,538us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale 2,116,460us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale 2,432,367us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale
Sales, net of gains (1,657,409)ibkc_SalesOfMortgageLoansHeldForSale (2,255,493)ibkc_SalesOfMortgageLoansHeldForSale (2,317,905)ibkc_SalesOfMortgageLoansHeldForSale
Other (6,499)ibkc_MortgageLoansHeldForSaleOther    
Balance at end of period $ 140,072us-gaap_LoansHeldForSaleMortgages $ 128,442us-gaap_LoansHeldForSaleMortgages $ 267,475us-gaap_LoansHeldForSaleMortgages