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Junior Subordinated Interest Deferrable Debentures (Details) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2025 USD ($) item | Dec. 31, 2024 USD ($)
Junior Subordinated Interest Deferrable Debentures |  |
Number of statutory business trusts issuing trust preferred securities | item | 4 |
Junior subordinated deferrable interest debentures | $ | $ 108,868,000 | $ 108,868,000
Maximum number of consecutive quarterly period | item | 20 |
Percentage of capital securities issued by trust qualifying as Tier I capital, maximum | 25.00% |
Percentage of capital securities issued by trust qualifying as Tier II capital, minimum | 25.00% |
Capital securities issued by the trust, qualifying as Tier I capital | $ | $ 108,868,000 | $ 108,868,000

X
- Definition Represents capital securities issued by trust that is qualified as Tier I capital as on the reporting date. + References No definition available. + Details
Name: | iboc_CapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Namespace Prefix: | iboc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the maximum number of consecutive quarterly periods available to the entity for deferring the payment of interest on Trusts VIII, IX, X, XI and XII. + References No definition available. + Details
Name: | iboc_MaximumNumberOfConsecutiveQuarterlyPeriodsAvailableForDeferralOfPaymentOfInterest
Namespace Prefix: | iboc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the maximum percentage of capital securities issued by the trust qualifying as Tier I capital on an aggregate basis. + References No definition available. + Details
Name: | iboc_MaximumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Namespace Prefix: | iboc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the minimum percentage of capital securities issued by the trust qualifying as Tier II capital on an aggregate basis. + References No definition available. + Details
Name: | iboc_MinimumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierTwoCapital
Namespace Prefix: | iboc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of statutory business trusts formed for the purpose of issuing trust preferred securities. + References No definition available. + Details
Name: | iboc_NumberOfStatutoryBusinessTrustsIssuingPreferredSecurities
Namespace Prefix: | iboc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)(a)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
Name: | us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_SubordinatedBorrowingsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration