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Allowance for Probable Loan Losses (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	$ 73,750us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment	$ 76,171us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Loans individually evaluated for impairment, Allowance	13,724us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1	12,221us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Loans collectively evaluated for impairment, Recorded Investment	5,704,992us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment	5,603,074us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Loans collectively evaluated for impairment, Allowance	52,527us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment	52,607us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	41,255us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember	40,175us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember
Loans individually evaluated for impairment, Allowance	10,790us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember	9,112us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember
Loans collectively evaluated for impairment, Recorded Investment	1,052,028us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember	1,049,311us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember
Loans collectively evaluated for impairment, Allowance	12,729us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember	13,240us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_CommercialLoanMember
Commercial real estate: other construction and land development
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	9,932us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember	10,876us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
Loans individually evaluated for impairment, Allowance	1,956us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember	1,890us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
Loans collectively evaluated for impairment, Recorded Investment	1,512,639us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember	1,404,101us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
Loans collectively evaluated for impairment, Allowance	12,413us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember	11,065us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
Commercial real estate: farmland and commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	13,467us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember	14,166us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember
Loans individually evaluated for impairment, Allowance	978us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember	1,219us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember
Loans collectively evaluated for impairment, Recorded Investment	1,882,526us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember	1,887,233us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember
Loans collectively evaluated for impairment, Allowance	16,998us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember	17,464us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember
Commercial real estate: multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	841us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember	835us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember
Loans collectively evaluated for impairment, Recorded Investment	104,405us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember	115,864us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember
Loans collectively evaluated for impairment, Allowance	840us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember	846us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_CommercialRealEstateMultifamilyReceivableMember
Residential: first lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	5,293us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember	5,840us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember
Loans collectively evaluated for impairment, Recorded Investment	418,515us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember	416,186us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember
Loans collectively evaluated for impairment, Allowance	3,573us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember	3,589us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialFirstMortgageFinancingReceivableMember
Residential: junior lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	1,369us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember	2,895us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember
Loans collectively evaluated for impairment, Recorded Investment	491,761us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember	485,405us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember
Loans collectively evaluated for impairment, Allowance	4,261us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember	4,683us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ResidentialSecondMortgageFinancingReceivableMember
Consumer Loan
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	1,188us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember	1,384us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember
Loans collectively evaluated for impairment, Recorded Investment	58,066us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember	59,753us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember
Loans collectively evaluated for impairment, Allowance	644us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember	660us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ConsumerLoanMember
Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans individually evaluated for impairment, Recorded Investment	405us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ForeignReceivablesMember
Loans collectively evaluated for impairment, Recorded Investment	185,052us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ForeignReceivablesMember	185,221us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ForeignReceivablesMember
Loans collectively evaluated for impairment, Allowance	$ 1,069us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ForeignReceivablesMember	$ 1,060us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = iboc_ForeignReceivablesMember

X

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

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