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Financial Instruments with Off-Statement of Condition Risk and Concentrations of Credit Risk (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Maximum |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Credit risk commitment, maximum expiration date | 1 year |
Commitments to extend credit |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Outstanding amount of financial instruments whose, contract amounts represent credit risks | $ 2,935,768,000 |
Credit card lines |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Outstanding amount of financial instruments whose, contract amounts represent credit risks | 21,114,000 |
Standby letters of credit |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Outstanding amount of financial instruments whose, contract amounts represent credit risks | 111,229,000 |
Guarantee obligations, maximum exposure | 111,229,000 |
Commercial letters of credit |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Outstanding amount of financial instruments whose, contract amounts represent credit risks | 8,073,000 |
Unsecured letters of credit |  |
Financial amounts of instruments, whose contract amounts represent credit risks |  |
Unsecured letters of credit | $ 42,729,000 | $ 35,409,000

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- Definition Represents the maximum fixed expiration date upon which commitments are set to expire. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611 + Details
Name: | us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_LineOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=iboc_CommercialLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |