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Acquisition and Disposition Activity - Schedule of Branch Closure Costs (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Business Combinations [Abstract]    
Employee termination $ 299ibkc_EmployeeTerminationAndOtherCosts $ 477ibkc_EmployeeTerminationAndOtherCosts
Accelerated depreciation 1,033ibkc_AcceleratedDepreciation 576ibkc_AcceleratedDepreciation
Contract termination 659ibkc_ContractTerminationExpenses 20ibkc_ContractTerminationExpenses
Impairment 4,941ibkc_ImpairmentOfBranchesScheduledToClose 2,743ibkc_ImpairmentOfBranchesScheduledToClose
Total $ 6,932ibkc_BusinessDispositionCostOfDisposedEntityTransactionCosts $ 3,816ibkc_BusinessDispositionCostOfDisposedEntityTransactionCosts