Fair Value Measurements - Summary of Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Mortgage Loans Held for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Mortgage loans held for sale, at fair value
Aggregate Fair Value	$ 213,357	$ 107,734
Aggregate Unpaid Principal	207,481	104,345
Aggregate Fair Value Less Unpaid Principal	5,876	3,389
Mortgage loans held for investment, at fair value
Aggregate Fair Value	2,792	3,143
Aggregate Unpaid Principal	2,922	3,595
Aggregate Fair Value Less Unpaid Principal	$ (130)	$ (452)

- Definition

Represents the unpaid principal balance of mortgage loans held for sale at the end of the period for those loans accounted for at fair value under the fair value option.

+ References

No definition available.

+ Details

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

- Definition

Represents the unpaid principal balance of mortgage loans held for investment at the end of the period for those loans accounted for at fair value under the fair value option.

+ References

No definition available.

+ Details

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

- Definition

Mortgages Held For Investment Fair Value In Excess Of Unpaid Principal Balance

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of mortgage loans held-for-investment.

+ References

No definition available.

+ Details

Name:	ibkc_MortgagesHeldForInvestmentatFairValueAbstract
Namespace Prefix:	ibkc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgages Held For Sale, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	ibkc_MortgagesHeldForSaleFairValueDisclosureAbstract
Namespace Prefix:	ibkc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate difference between fair value and unpaid principal of mortgage loans held for sale recorded at fair value at the end of the period under the fair value option.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of mortgage loans held-for-sale.

+ References

No definition available.

+ Details

Name:	us-gaap_MortgagesHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant