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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
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Fair Value Disclosures [Abstract]    
Mortgage loans held for sale, at fair value, Aggregate Fair Value $ 194,816us-gaap_MortgagesHeldForSaleFairValueDisclosure $ 139,950us-gaap_MortgagesHeldForSaleFairValueDisclosure
Mortgage loans held for sale, at fair value, Aggregate Unpaid Principal 187,455ibkc_MortgageLoanHeldForSaleUnpaidPrincipalBalance 134,639ibkc_MortgageLoanHeldForSaleUnpaidPrincipalBalance
Mortgage loans held for sale, at fair value, Aggregate Fair Value Less Unpaid Principal $ 7,361ibkc_MortgagesHeldForSaleFairValueInExcessOfUnpaidPrincipalBalance $ 5,311ibkc_MortgagesHeldForSaleFairValueInExcessOfUnpaidPrincipalBalance