Quarterly Results of Operations and Selected Cash Flow Data (Unaudited) - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2014	Jun. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Mortgage income	$ 1.5	$ 4.2

The effect of the correction of an error and other prior period adjustments recorded on each financial statement line item for each prior period presented.

No definition available.

Name:	ibkc_ErrorCorrectionAndPriorPeriodAdjustment
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration