Commitments and Contingencies (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Fair value of guarantees under commercial and standby letters of credit	$ 1.5	$ 1.3

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