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Long-Term Debt - Schedule of Long-Term Debt (Parenthetical) (Detail)
 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]    
3-month LIBOR rate 0.26%ibkc_ThreeMonthLondonInterBankOfferRate 0.25%ibkc_ThreeMonthLondonInterBankOfferRate
Statutory Trust I, 3 Month LIBOR Plus 3.25% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 3.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustOneThreeMonthLiborPlusThreePointTwoFivePercentageMember		  
Statutory Trust II, 3 Month LIBOR Plus 3.15% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 3.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustTwoThreeMonthLiborPlusThreePointOneFivePercentageMember		  
Statutory Trust III, 3 Month LIBOR Plus 2.00% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustThreeThreeMonthLiborPlusTwoPointZeroZeroPercentageMember		  
Statutory Trust IV, 3 Month LIBOR Plus 1.60% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 1.60%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustFourThreeMonthLiborPlusOnePointSixZeroPercentageMember		  
American Horizons Statutory Trust I, 3 Month LIBOR Plus 3.15% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 3.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_AmericanHorizonsStatutoryTrustIThreeMonthLiborPlusThreePointOneFivePercentageMember		  
Statutory Trust V, 3 Month LIBOR Plus 1.435% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 1.435%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustFiveThreeMonthLiborPlusOnePointFourThreeFivePercentageMember		  
Statutory Trust VI, 3 Month LIBOR Plus 2.75% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 2.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustSixThreeMonthLiborPlusTwoPointSevenFivePercentageMember		  
Statutory Trust VII, 3 Month LIBOR Plus 2.54% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 2.54%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustSevenThreeMonthLiborPlusTwoPointFiveFourPercentageMember		  
Statutory Trust VIII, 3 Month LIBOR Plus 3.50% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 3.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_StatutoryTrustEightThreeMonthLiborPlusThreePointFiveZeroPercentageMember		  
OMNI Trust I, 3 Month LIBOR Plus 3.30% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 3.30%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_OmniTrustOneThreeMonthLiborPlusThreePointThreeZeroPercentageMember		  
OMNI Trust II, 3 Month LIBOR Plus 2.79% [Member]    
Debt Instrument [Line Items]    
LIBOR plus rate 2.79%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_SubordinatedBorrowingAxis
= ibkc_OmniTrustThreeMonthLiborPlusTwoPointSevenNinePercentageMember		  
0.724% to 7.04% [Member]    
Debt Instrument [Line Items]    
Long-term debt, fixed interest, percentage rate minimum 0.724%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= ibkc_ZeroPointSevenTwoFourToSevenPointZeroFourPercentageFixedMember		  
Long-term debt, fixed interest, percentage rate maximum 7.04%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= ibkc_ZeroPointSevenTwoFourToSevenPointZeroFourPercentageFixedMember		  
0.50% to 5.00% Fixed [Member]    
Debt Instrument [Line Items]    
Long-term debt, fixed interest, percentage rate minimum 0.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= ibkc_ZeroPointFiveZeroToFivePointZeroZeroPercentFixedMember		  
Long-term debt, fixed interest, percentage rate maximum 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= ibkc_ZeroPointFiveZeroToFivePointZeroZeroPercentFixedMember		  
Minimum [Member] | 0.50% to 5.00% Fixed [Member]    
Debt Instrument [Line Items]    
Debt instrument maturity date period, (in years) P7Y  
Maximum [Member] | 0.50% to 5.00% Fixed [Member]    
Debt Instrument [Line Items]    
Debt instrument maturity date period, (in years) P40Y