Transfers and Servicing of Financial Assets (Including Mortgage Banking Activity) (Detail) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016
Transfers and Servicing [Abstract]
Unpaid principal balances of loans serviced	$ 1.3	$ 1.0

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