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Acquisition and Disposition Activity - Schedule of Assets Acquired under Business Acquisitions (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Jan. 17, 2014
Dec. 31, 2013
Dec. 31, 2012
Feb. 24, 2014
May 01, 2014
Business Acquisition [Line Items]            
Goodwill $ 517,526us-gaap_Goodwill $ 8,600us-gaap_Goodwill $ 401,872us-gaap_Goodwill $ 401,872us-gaap_Goodwill    
Title Company LLC [Member]            
Business Acquisition [Line Items]            
Goodwill 221us-gaap_Goodwill
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
      200us-gaap_Goodwill
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
 
Non-compete agreement 63ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNonCompeteAgreement
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
         
Title plant 14ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTitlePlant
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
         
Other intangible assets 75ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherIntangiblesAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
         
Other assets 3us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
         
Total Assets 376us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_TitleCompanyLlcMember
         
Louisiana Abstract and Title Llc [Member]            
Business Acquisition [Line Items]            
Goodwill 155us-gaap_Goodwill
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
        200us-gaap_Goodwill
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
Non-compete agreement 100ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNonCompeteAgreement
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
         
Title plant 9ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTitlePlant
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
         
Other intangible assets 130ibkc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherIntangiblesAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
         
Other assets 6us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember
         
Total Assets $ 400us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_EquityInterestIssuedOrIssuableByTypeAxis
= ibkc_LouisianaAbstractAndTitleLlcMember