Transfers and Servicing of Financial Assets (Including Mortgage Banking Activity) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Transfers and Servicing [Abstract]
Unpaid principal balances of loans serviced	$ 888.4	$ 533.8

Represents the unpaid principal balance of loans serviced for others at the balance sheet date.

No definition available.

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