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Capital Ratios and Other Regulatory Matters - Additional Information (Detail)
 3 Months Ended
Mar. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]  
Increased Minimum Tier 1 capital ("CET1") to risk-weighted assets 4.50%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Minimum Tier 1 capital buffer requirement 2.50%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Minimum capital conservation buffer 0.625%ibkc_TierOneRiskBasedCapitalRequiredMinimumCapitalConversionBuffer
Capital conservation buffer ratio 2.50%ibkc_TierOneRiskBasedCapitalRequiredCapitalConservationBufferRatio
Minimum [Member]  
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]  
Increased Minimum Tier 1 capital ("CET1") to risk-weighted assets 4.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]  
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]  
Increased Minimum Tier 1 capital ("CET1") to risk-weighted assets 6.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_RangeAxis
= us-gaap_MaximumMember