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Fair Value Measurements - Summary of Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Mortgage Loans Held for Sale (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value Disclosures [Abstract]    
Mortgage loans held for sale, at fair value, Aggregate Fair Value $ 139,950us-gaap_MortgagesHeldForSaleFairValueDisclosure $ 97,273us-gaap_MortgagesHeldForSaleFairValueDisclosure
Mortgage loans held for sale, at fair value, Aggregate Unpaid Principal 134,639ibkc_MortgageLoanHeldForSaleUnpaidPrincipalBalance 96,875ibkc_MortgageLoanHeldForSaleUnpaidPrincipalBalance
Mortgage loans held for sale, at fair value, Aggregate Fair Value Less Unpaid Principal $ 5,311ibkc_MortgagesHeldForSaleFairValueInExcessOfUnpaidPrincipalBalance $ 398ibkc_MortgagesHeldForSaleFairValueInExcessOfUnpaidPrincipalBalance