Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash as collateral for derivative transactions	$ 21.8	$ 11.5

Cash posted as collateral for derivatives in liability position.

No definition available.

Name:	ibkc_CashPostedAsCollateralForDerivativesInLiabilityPosition
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration