Loans and Leases - Schedule of Subsequently Defaulted TDRs (Detail) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan	Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable
Threshold period for loans in default (in days)			30 days
TDRs \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	82	87	182	145
Pre-modification Outstanding Recorded Investment	$ 20,999	$ 27,761	$ 46,932	$ 58,105
Post-modification Outstanding Recorded Investment	$ 20,652	$ 20,108	$ 37,409	$ 46,691
Number of loans \| SecurityLoan	96	61	138	126
Recorded investment	$ 12,023	$ 5,482	$ 14,923	$ 22,694
TDRs \| Commercial Loans \| Real estate - construction \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan			1	1
Pre-modification Outstanding Recorded Investment			$ 39	$ 1,950
Post-modification Outstanding Recorded Investment			$ 37	$ 1,013
Number of loans \| SecurityLoan	1	0	1	0
Recorded investment	$ 37	$ 0	$ 37	$ 0
TDRs \| Commercial Loans \| Real estate - owner-occupied \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	4	5	6	7
Pre-modification Outstanding Recorded Investment	$ 339	$ 2,230	$ 1,435	$ 12,921
Post-modification Outstanding Recorded Investment	$ 338	$ 2,201	$ 1,402	$ 11,290
Number of loans \| SecurityLoan	4	5	6	6
Recorded investment	$ 1,232	$ 461	$ 1,232	$ 9,455
TDRs \| Commercial Loans \| Real estate - non-owner-occupied \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	7	5	13	14
Pre-modification Outstanding Recorded Investment	$ 4,687	$ 823	$ 7,756	$ 1,912
Post-modification Outstanding Recorded Investment	$ 4,628	$ 780	$ 7,651	$ 1,854
Number of loans \| SecurityLoan	11	9	17	15
Recorded investment	$ 2,341	$ 1,448	$ 2,913	$ 1,813
TDRs \| Commercial Loans \| Commercial and industrial \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	19	16	38	32
Pre-modification Outstanding Recorded Investment	$ 13,757	$ 21,103	$ 29,785	$ 35,338
Post-modification Outstanding Recorded Investment	$ 13,607	$ 13,618	$ 20,670	$ 26,684
Number of loans \| SecurityLoan	17	8	25	21
Recorded investment	$ 5,475	$ 1,437	$ 6,152	$ 7,100
TDRs \| Residential mortgage loans \| Residential mortgage \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	10	7	19	7
Pre-modification Outstanding Recorded Investment	$ 847	$ 688	$ 1,687	$ 688
Post-modification Outstanding Recorded Investment	$ 836	$ 633	$ 1,669	$ 633
Number of loans \| SecurityLoan	12	7	21	10
Recorded investment	$ 1,012	$ 775	$ 1,597	$ 1,081
TDRs \| Consumer and Other Loans \| Consumer - Home equity \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	22	19	54	33
Pre-modification Outstanding Recorded Investment	$ 1,159	$ 2,361	$ 5,312	$ 4,170
Post-modification Outstanding Recorded Investment	$ 1,056	$ 2,350	$ 5,137	$ 4,133
Number of loans \| SecurityLoan	17	11	27	26
Recorded investment	$ 1,411	$ 1,129	$ 2,401	$ 2,242
TDRs \| Consumer and Other Loans \| Consumer - Other \| TDRs Occurring during the Period
Accounts, Notes, Loans and Financing Receivable
Number of Loans \| SecurityLoan	20	35	51	51
Pre-modification Outstanding Recorded Investment	$ 210	$ 556	$ 918	$ 1,126
Post-modification Outstanding Recorded Investment	$ 187	$ 526	$ 843	$ 1,084
Number of loans \| SecurityLoan	34	21	41	48
Recorded investment	$ 515	$ 232	$ 591	$ 1,003

Threshold Period for Loans in Default

No definition available.

Name:	ibkc_ThresholdPeriodforLoansinDefault
Namespace Prefix:	ibkc_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=ibkc_TroubledDebtRestructuringsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TroubledDebtRestructuringDebtorCurrentPeriodAxis=ibkc_TroubledDebtRestructuringOccurringDuringCurrentPeriodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ibkc_CommercialRealEstateOwneroccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ibkc_CommercialRealEstateNonowneroccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_PrimeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ibkc_ConsumerOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: