Other Real Estate Owned - Schedule of Other Real Estate Owned Segregated into Non-Covered and Covered Properties (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Statement of Financial Position [Abstract]
Covered other real estate owned	$ 9.6

Carrying amount of other real estate covered by loss share agreements that will lose loss share coverage within the next 12 months.

No definition available.

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