The amount of loans approved and closed as part of the lending program.

No definition available.

Name:	iboc_AmountOfLoansApprovedAndClosed
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The term of financing receivables.

+ References

No definition available.

+ Details

Name:	iboc_FinancingReceivableTerm
Namespace Prefix:	iboc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable with some degree of payment deferrals.

+ References

No definition available.

+ Details

Name:	iboc_FinancingReceivableWithPaymentDeferrals
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The contractual interest rate on financing receivable.

+ References

No definition available.

+ Details

Name:	iboc_FinancingReceivablesStatedInterestRate
Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the period of time the PPP loans are intended to support payroll and certain other costs to help businesses remain viable and allow their employees to pay their bills.

+ References

No definition available.

+ Details

Name:	iboc_MaximumPeriodOfTimeLoansSupportPayrollAndOtherCosts
Namespace Prefix:	iboc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of loans approved and closed as part of the lending program.

+ References

No definition available.

+ Details

Name:	iboc_NumberOfLoansApprovedAndClosed
Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

No definition available.

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ 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=121611835&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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

- Definition

Threshold period for when financing receivable is considered past due to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447

+ Details

Name:	us-gaap_FinancingReceivableThresholdPeriodPastDueWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	srt_StatementScenarioAxis=iboc_LoanScenarioOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=iboc_LoanScenarioTwoMember
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_GeographicDistributionAxis=us-gaap_GeographicDistributionDomesticMember
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:

Allowance for Credit Losses (Impaired Loans) (Details)	3 Months Ended
Allowance for Credit Losses (Impaired Loans) (Details)	Mar. 31, 2021 USD ($)	May 01, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
Loan loss allowances, impaired financing receivable
Total trouble debt restructuring	$ 8,913,000		$ 5,821,000
Amount of loans with some degree of payment deferral		$ 602,000,000
Period of charge off for past due unsecured commercial loans	90 days
Paycheck protection program loans (COVID-19 program)
Loan loss allowances, impaired financing receivable
Period of time PPP loans cover payroll and certain other costs	168 days
Interest rate on receivable	1.00%
Number of loans \| loan		3,954
Amount of loans outstanding		$ 374,981,000
Paycheck protection program loans (COVID-19 program) \| Loans made prior to June 5, 2020
Loan loss allowances, impaired financing receivable
Term of receivable (in years)	2 years
Paycheck protection program loans (COVID-19 program) \| Loans made after June 5, 2020
Loan loss allowances, impaired financing receivable
Term of receivable (in years)	5 years
Real estate - mortgage \| Domestic \| Residential First Mortgage Financing Receivable [Member]
Loan loss allowances, impaired financing receivable
Total trouble debt restructuring	$ 6,869,000		4,078,000
Real estate - mortgage \| Domestic \| Residential Junior Lien
Loan loss allowances, impaired financing receivable
Total trouble debt restructuring	469,000		521,000
Consumer \| Domestic
Loan loss allowances, impaired financing receivable
Total trouble debt restructuring	938,000		989,000
Foreign \| Foreign
Loan loss allowances, impaired financing receivable
Total trouble debt restructuring	$ 637,000		$ 233,000