Goodwill and Other Intangible Assets - Narrative (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Title plant assets	$ 6.8	$ 6.8

Balance of title plant assets recorded as of the balance sheet date.

No definition available.

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