Derivative Instruments and Other Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash as collateral for derivative transactions	$ 11.5	$ 5.0
Fair value of derivatives	$ 0.4ibkc_FairValueOfDerivatives

Total fair value of derivatives at the end of the current period that are scheduled to mature within the next 12 months

No definition available.

Name:	ibkc_FairValueOfDerivatives
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant