Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Fair value of guarantees under commercial and standby letters of credit	$ 1.3ibkc_LetterOfCreditFairValueDisclosure	$ 1.3ibkc_LetterOfCreditFairValueDisclosure

Represents the estimated fair value of letters of credit outstanding at the balance sheet date.

No definition available.

Name:	ibkc_LetterOfCreditFairValueDisclosure
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant