Allowance for Probable Loan Losses (Impairment By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 32,492	$ 63,204
Loans Individually Evaluated for Impairment, Allowance	700	434
Loans Collectively Evaluated for Impairment, Recorded Investment	6,442,649	6,284,968
Loans Collectively Evaluated for Impairment, Allowance	59,954	67,253
Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	17,236	17,947
Loans Individually Evaluated for Impairment, Allowance	584	300
Loans Collectively Evaluated for Impairment, Recorded Investment	1,061,329	1,068,520
Loans Collectively Evaluated for Impairment, Allowance	12,742	27,605
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	2,130	2,455
Loans Individually Evaluated for Impairment, Allowance	116	116
Loans Collectively Evaluated for Impairment, Recorded Investment	1,876,310	1,681,095
Loans Collectively Evaluated for Impairment, Allowance	14,988	11,559
Commercial Real Estate: Farmland and Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	4,157	33,123
Loans Individually Evaluated for Impairment, Allowance		18
Loans Collectively Evaluated for Impairment, Recorded Investment	1,977,397	2,010,162
Loans Collectively Evaluated for Impairment, Allowance	19,365	16,645
Commercial Real Estate: Multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	586	476
Loans Collectively Evaluated for Impairment, Recorded Investment	186,759	192,440
Loans Collectively Evaluated for Impairment, Allowance	1,436	1,109
Residential First Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	6,173	6,852
Loans Collectively Evaluated for Impairment, Recorded Investment	420,373	425,925
Loans Collectively Evaluated for Impairment, Allowance	3,186	2,950
Residential Junior Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	740	723
Loans Collectively Evaluated for Impairment, Recorded Investment	720,346	700,025
Loans Collectively Evaluated for Impairment, Allowance	6,907	6,103
Consumer
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	1,164	1,281
Loans Collectively Evaluated for Impairment, Recorded Investment	45,482	48,262
Loans Collectively Evaluated for Impairment, Allowance	439	440
Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	306	347
Loans Collectively Evaluated for Impairment, Recorded Investment	154,653	158,539
Loans Collectively Evaluated for Impairment, Allowance	$ 891	$ 842

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_CommercialRealEstateFarmlandAndCommercialReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_CommercialRealEstateMultifamilyReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_ResidentialFirstMortgageFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_ResidentialSecondMortgageFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=iboc_ForeignReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: