XML 65 R75.htm IDEA: XBRL DOCUMENT v2.4.1.9
Investment Securities - Additional Information (Detail) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Security
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Security
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Security
Dec. 31, 2013
Security
Dec. 31, 2012
Bonds
Net Investment Income [Line Items]                      
Pledged securities, carrying values $ 1,400,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue       $ 1,500,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue       $ 1,400,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue $ 1,500,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue  
Number of debt securities, held 112ibkc_NumberOfDebtSecuritiesHeldUnrealizedLosses       207ibkc_NumberOfDebtSecuritiesHeldUnrealizedLosses       112ibkc_NumberOfDebtSecuritiesHeldUnrealizedLosses 207ibkc_NumberOfDebtSecuritiesHeldUnrealizedLosses  
Unrealized losses on debt securities, percent of amortized cost 1.31%ibkc_PercentageOfUnrealizedLossesOfDebtSecuritiesWithAmortizedCost       3.23%ibkc_PercentageOfUnrealizedLossesOfDebtSecuritiesWithAmortizedCost       1.31%ibkc_PercentageOfUnrealizedLossesOfDebtSecuritiesWithAmortizedCost 3.23%ibkc_PercentageOfUnrealizedLossesOfDebtSecuritiesWithAmortizedCost  
Unrated revenue municipal bond                     1ibkc_OtherThanTemporaryImpairmentUnratedRevenueMunicipalBond
Impairment of par value bond, percentage                 50.00%ibkc_ImpairmentOfParValueBondPercentage    
Gain (loss) on sale of available for sale investments $ 164,000us-gaap_GainLossOnSaleOfInvestments $ 582,000us-gaap_GainLossOnSaleOfInvestments $ 8,000us-gaap_GainLossOnSaleOfInvestments $ 19,000us-gaap_GainLossOnSaleOfInvestments $ 19,000us-gaap_GainLossOnSaleOfInvestments $ 13,000us-gaap_GainLossOnSaleOfInvestments $ (57,000)us-gaap_GainLossOnSaleOfInvestments $ 2,359,000us-gaap_GainLossOnSaleOfInvestments $ 771,000us-gaap_GainLossOnSaleOfInvestments $ 2,277,000us-gaap_GainLossOnSaleOfInvestments $ 3,739,000us-gaap_GainLossOnSaleOfInvestments
Obligations of State and Political Obligations [Member]                      
Net Investment Income [Line Items]                      
Number of securities, continuous loss position, over twelve months 0ibkc_NumberOfAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLonger
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		               0ibkc_NumberOfAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLonger
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember