Income Taxes Income Taxes (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Income Tax Disclosure [Abstract]
Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Deferred Tax Asset, Income Tax Expense (Benefit)	$ 51.0

Amount of income tax expense (benefit) for remeasurement of deferred tax asset from change in tax rate pursuant to Tax Cuts and Jobs Act of 2017.

No definition available.

Name:	ibkc_TaxCutsandJobsActof2017ChangeinTaxRateDeferredTaxAssetIncomeTaxExpenseBenefit
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration