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Fair Value (Details 3) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Assets: |  |
Impaired Loans | $ 65,551,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure | $ 64,585,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure
Non-financial assets: |  |
Other real estate owned | 69,872,000 iboc_OtherRealEstateAndForeclosedAssetsInUse | 85,879,000 iboc_OtherRealEstateAndForeclosedAssetsInUse
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Charges to allowance for probable loan losses in connection with other real estate owned | 367,000 iboc_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecreaseLoansTransferredToOtherRealEstateOwned | 402,000 iboc_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecreaseLoansTransferredToOtherRealEstateOwned
Adjustment to fair value in connection with other real estate owned | 597,000 iboc_AllowanceForLoanLeaseLossesWriteOffsPriorToForeclosure | 1,204,000 iboc_AllowanceForLoanLeaseLossesWriteOffsPriorToForeclosure
Impaired commercial collateral dependent loans | 65,551,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure | 64,585,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure
Impaired commercial collateral dependent receivables appraisals to determine fair value within immediately preceding twelve months | 52,092,000 iboc_ImpairedCommercialCollateralFinancingReceivableAppraisalsToDetermineFairValue | 50,346,000 iboc_ImpairedCommercialCollateralFinancingReceivableAppraisalsToDetermineFairValue
Impaired collateral dependent commercial loans with an internal evaluation completed in the last twelve months | 5,307,000 iboc_ImpairedCollateralDepndentCommercialLoansWithInternalEvaluationCompletedWithinLastTwelveMonths | 0 iboc_ImpairedCollateralDepndentCommercialLoansWithInternalEvaluationCompletedWithinLastTwelveMonths
Significant Unobservable Inputs (Level 3) | Bond meeting the original contract terms |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Minimum period of residential mortgage loan performance under original contract terms | 24 months | 24 months
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 7.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentRatesLowEndOfRange / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember | 7.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentRatesLowEndOfRange / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember
Default rate assumptions (as a percent) | 1.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDefaultRates / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember | 1.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDefaultRates / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember
Loss severity rate assumptions, first year (as a percent) | 25.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesForFirstYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember | 25.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesForFirstYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember
Estimated future principal prepayment rate assumption, discount rate (as a percent) | 13.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember | 13.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondMeetingOriginalContractTermsMember
Significant Unobservable Inputs (Level 3) | Bond not meeting the original contract terms |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 2.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentRatesLowEndOfRange / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 2.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentRatesLowEndOfRange / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Default rate assumptions (as a percent) | 4.50%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDefaultRates / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 4.50%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDefaultRates / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Loss severity rate assumptions, first year (as a percent) | 60.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesForFirstYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 60.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesForFirstYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Decrease in loss severity rates, following five years (as a percent) | 5.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDeclineInLossSeverityRatesForFollowingFifthYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 5.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsDeclineInLossSeverityRatesForFollowingFifthYear / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Loss severity rate, thereafter (as a percent) | 25.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesThereafter / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 25.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsLossSeverityRatesThereafter / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Estimated future principal prepayment rate assumption, discount rate (as a percent) | 13.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember | 13.00%iboc_FairValueMeasurementsDiscountedCashFlowModelAssumptionsFuturePrincipalPrepaymentDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = iboc_BondNotMeetingOriginalContractTermsMember
Measured on a non-recurring basis: |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Change in net provision, impaired loans | (1,557,000)iboc_ChangeInProvisionOfImpairedFinancingReceivableFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 13,229,000 iboc_ChangeInProvisionOfImpairedFinancingReceivableFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Change in net provision, other real estate owned | 597,000 iboc_ChangeInProvisionOfOtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 1,204,000 iboc_ChangeInProvisionOfOtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Measured on a non-recurring basis: | Assets/Liabilities Measured at Fair Value |  |
Assets: |  |
Impaired Loans | 29,501,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 28,391,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-financial assets: |  |
Other real estate owned | 6,112,000 iboc_OtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 16,329,000 iboc_OtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Impaired commercial collateral dependent loans | 29,501,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 28,391,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Measured on a non-recurring basis: | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Impaired Loans | 29,501,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 28,391,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Non-financial assets: |  |
Other real estate owned | 6,112,000 iboc_OtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | 16,329,000 iboc_OtherRealEstateAndForeclosedAssetsInUse / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Impaired commercial collateral dependent loans | $ 29,501,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | $ 28,391,000 iboc_ImpairedFinancingReceivableRecordedInvestmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember

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- Definition Amount of increase (decrease) of allowance reserve for credit losses in connection with loans transferred to other real estate owned. + References No definition available. + Details
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- Definition Reflects the amount of charge-off related to a loan prior to foreclosure. + References No definition available. + Details
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- Definition The estimated fair value of recorded investment related to change in provision for impaired financing receivables measured at fair value. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of change in Provision of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings. + References No definition available. + Details
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- Definition The decline in the loss severity rate assumptions for the following five years used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Default rate assumption used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition The discount rate assumption of estimated future principal prepayment rate assumption used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition The low end of range of estimated future principal prepayment rate assumption used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Loss severity rates assumption for the first year used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Loss severity rates assumption for the remaining years used in the discounted cash flow model for investments classified within level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition The minimum period where the underlying residential mortgage loans must perform under original contract terms as a basis for certain discounted cash flow assumptions to be used. + References No definition available. + Details
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- Definition The impaired collateral dependent commercial loans where an internal evaluation has been done in the last twelve months. + References No definition available. + Details
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- Definition Represents the impaired commercial collateral dependent financing receivables where the basis for fair value is an appraisal obtained in the last twelve months. + References No definition available. + Details
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- Definition The estimated fair value of the recorded impaired collateral measured on a non-recurring basis as of the reporting date. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings. + References No definition available. + Details
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