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Loss Sharing Agreements and FDIC Loss Share Receivable - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Business
Dec. 31, 2014
Receivables [Abstract]    
Percentage of covered loan and foreclosed real estate losses 80.00%ibkc_PercentageOfLoanLossesCoveredUnderLossShareAgreement  
Losses that exceed contractual thresholds 95.00%ibkc_PercentageAcquiredByFdicInFirstAmountExceedingLossesCoveredAssets  
Number of acquisitions 4ibkc_NumberOfBusinessesAcquiredWithIndemnificationAgreements  
Recovery period for covered assets 3 years  
FDIC loss share receivable $ 60,972us-gaap_FDICIndemnificationAsset $ 69,627us-gaap_FDICIndemnificationAsset