Allowance for Credit Losses - Impairment By Loan Class (Details) $ in Thousands	Jun. 30, 2024 USD ($) loan	Dec. 31, 2023 USD ($)
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 102,021	$ 47,061
Loans Individually Evaluated for Impairment, Allowance	9,441	12,291
Loans Collectively Evaluated for Impairment, Recorded Investment	8,165,806	8,011,900
Loans Collectively Evaluated for Impairment, Allowance	$ 139,168	144,778
Commercial real estate: farmland & commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Number of loans downgraded \| loan	1
Commercial real estate: multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Number of loans downgraded \| loan	2
Commercial, financial and agricultural \| Domestic \| Commercial.
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 1,377	30,872
Loans Individually Evaluated for Impairment, Allowance	400	7,971
Loans Collectively Evaluated for Impairment, Recorded Investment	1,583,800	1,597,358
Loans Collectively Evaluated for Impairment, Allowance	26,855	27,579
Commercial, financial and agricultural \| Domestic \| Commercial real estate: farmland & commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	51,915	299
Loans Individually Evaluated for Impairment, Allowance	5,541
Loans Collectively Evaluated for Impairment, Recorded Investment	2,815,477	2,793,254
Loans Collectively Evaluated for Impairment, Allowance	39,347	42,703
Commercial, financial and agricultural \| Domestic \| Commercial real estate: multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	38,645	96
Loans Collectively Evaluated for Impairment, Recorded Investment	278,489	380,743
Loans Collectively Evaluated for Impairment, Allowance	3,528	5,088
Obligations of states and political subdivisions \| Domestic \| Commercial Real Estate: other construction and land development
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	10,029	15,701
Loans Individually Evaluated for Impairment, Allowance	3,500	4,320
Loans Collectively Evaluated for Impairment, Recorded Investment	2,293,661	2,075,921
Loans Collectively Evaluated for Impairment, Allowance	51,340	50,971
Real estate - mortgage \| Domestic \| Residential: first lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	55	93
Loans Collectively Evaluated for Impairment, Recorded Investment	498,595	477,940
Loans Collectively Evaluated for Impairment, Allowance	5,806	5,812
Real estate - mortgage \| Domestic \| Residential Junior Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Collectively Evaluated for Impairment, Recorded Investment	469,142	460,868
Loans Collectively Evaluated for Impairment, Allowance	10,639	11,024
Consumer \| Domestic
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Collectively Evaluated for Impairment, Recorded Investment	46,954	45,121
Loans Collectively Evaluated for Impairment, Allowance	314	318
Foreign. \| Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Collectively Evaluated for Impairment, Recorded Investment	179,688	180,695
Loans Collectively Evaluated for Impairment, Allowance	$ 1,339	$ 1,283

The number of loans downgraded.

No definition available.

Name:	iboc_NumberOfLoansDowngraded
Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of allowance for credit loss on financing receivable evaluated on collective basis. Excludes financing receivable evaluated on individual basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.5.a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 30
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479391/326-20-30-2

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of allowance for credit loss on financing receivable evaluated on individual basis. Excludes financing receivable evaluated on collective basis.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount of financing receivable collectively evaluated for credit loss. Excludes financing receivable individually evaluated for credit loss.

+ References

+ Details

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

- Definition

Amount of financing receivable individually evaluated for credit loss. Excludes financing receivable collectively evaluated for credit loss.

+ References

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_GeographicDistributionAxis=us-gaap_GeographicDistributionDomesticMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_LienCategoryAxis=us-gaap_SeniorLienMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LienCategoryAxis=us-gaap_JuniorLienMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_GeographicDistributionAxis=us-gaap_GeographicDistributionForeignMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: