Derivative Instruments and Other Hedging Activities - Effect of Hedging Instruments on Consolidated Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Designated as Hedging Instruments \| Cash Flow Hedging \| Interest rate contracts \| Interest expense
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	$ (162)	$ (22)	$ (467)	$ (178)
Derivative, included component, gain (loss) recognized in earnings	102	(22)	212	(178)
Amount of gain (loss) recognized in income on derivatives (amount excluded from effectiveness testing)	(264)	0	(679)	0
Derivative instruments, gain (loss) recognized in income, ineffective portion and amount excluded from effectiveness testing, net	0	0	0	0
Derivative instruments, gain (loss) recognized in income, ineffective portion and amount excluded from effectiveness testing, included component net	0	0	0	0
Derivative instruments, gain (loss) recognized in income, ineffective portion and amount excluded from effectiveness testing, excluding component, net	0	0	0	0
Not Designated as Hedging Instruments
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	390	2,850	4,266	10,142
Not Designated as Hedging Instruments \| Interest rate contracts \| Commission income
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	4,067	2,586	10,761	5,093
Not Designated as Hedging Instruments \| Foreign exchange contracts \| Commission income
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	8	7	14	17
Not Designated as Hedging Instruments \| Forward sales contracts \| Mortgage income
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	(1,765)	948	(8,085)	4,563
Not Designated as Hedging Instruments \| Written and purchased options \| Mortgage income
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	(1,848)	(692)	1,675	473
Not Designated as Hedging Instruments \| Other contracts \| Commission income
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	(72)	1	(99)	(4)
OCI \| Designated as Hedging Instruments \| Cash Flow Hedging \| Interest rate contracts
Derivative Instruments and Hedging Activities Disclosures
Amount of gain (loss) recognized	(264)	818	1,992	4,762
Derivative, included component, gain (loss) recognized in earnings	312	818	3,196	4,762
Amount of gain (loss) recognized in income on derivatives (amount excluded from effectiveness testing)	$ (576)	$ 0	$ (1,204)	$ 0

Derivative, Included Component, Gain (Loss) Recognized in Earnings

No definition available.

Name:	ibkc_DerivativeIncludedComponentGainLossRecognizedinEarnings
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Excluding Compnonent, Net

+ References

No definition available.

+ Details

Name:	ibkc_DerivativeInstrumentsGainLossRecognizedinIncomeIneffectivePortionandAmountExcludedfromEffectivenessTestingExcludingCompnonentNet
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Included Component Net

+ References

No definition available.

+ Details

Name:	ibkc_DerivativeInstrumentsGainLossRecognizedinIncomeIneffectivePortionandAmountExcludedfromEffectivenessTestingIncludedComponentNet
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) from excluded component of derivative hedge, recognized in earnings. Excludes recognition under systematic and rational method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 20
-Section 45
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109974929&loc=SL110061172-113977

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

+ Details

Name:	us-gaap_DerivativeExcludedComponentGainLossRecognizedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1),(2)
-URI http://asc.fasb.org/extlink&oid=109977999&loc=d3e76258-113986

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=ibkc_CommissionIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=ibkc_MortgageIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ibkc_WrittenAndPurchasedOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OtherComprehensiveIncomeLocationAxis=us-gaap_OtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: