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Junior Subordinated Deferrable Interest Debentures (Details) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 USD ($) item | Dec. 31, 2017 USD ($)
Junior Subordinated Interest Deferrable Debentures |  |
Number of statutory business trusts issuing trust preferred securities | item | 6 |
Junior subordinated deferrable interest debentures | $ | $ 160,416,000 | $ 160,416,000
Maximum number of consecutive quarterly period available for deferral of interest payment on Trusts VI, VIII, IX, X, XI and XII | 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 | $ | $ 160,416,000 | $ 160,416,000

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 VI, VII, VIII, IX, X, XI and XII. + References No definition available. + Details
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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
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Namespace Prefix: | iboc_
Data Type: | num: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: | num: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 -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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