Allowance for Probable Loan Losses (Aging By Loan Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing receivable recorded investment
Past due	$ 53,434	$ 61,400
Past due 90 days or Greater and Still Accruing	8,981	5,226
Loans, current	5,956,069	5,903,288
Total loans	6,009,503	5,964,688
30 to 59 Days
Financing receivable recorded investment
Past due	14,160	17,299
60 to 89 Days
Financing receivable recorded investment
Past due	4,156	6,327
90 Days or Greater
Financing receivable recorded investment
Past due	35,118	37,774
Commercial
Financing receivable recorded investment
Past due	24,448	26,033
Past due 90 days or Greater and Still Accruing	781	392
Loans, current	839,745	883,634
Total loans	864,193	909,667
Commercial \| 30 to 59 Days
Financing receivable recorded investment
Past due	3,048	4,081
Commercial \| 60 to 89 Days
Financing receivable recorded investment
Past due	796	829
Commercial \| 90 Days or Greater
Financing receivable recorded investment
Past due	20,604	21,123
Commercial Real Estate: Other Construction and Land Development
Financing receivable recorded investment
Past due	4,727	6,354
Past due 90 days or Greater and Still Accruing	6	9
Loans, current	1,773,555	1,710,521
Total loans	1,778,282	1,716,875
Commercial Real Estate: Other Construction and Land Development \| 30 to 59 Days
Financing receivable recorded investment
Past due	2,659	1,502
Commercial Real Estate: Other Construction and Land Development \| 60 to 89 Days
Financing receivable recorded investment
Past due	375	396
Commercial Real Estate: Other Construction and Land Development \| 90 Days or Greater
Financing receivable recorded investment
Past due	1,693	4,456
Commercial Real Estate: Farmland & Commercial
Financing receivable recorded investment
Past due	8,929	12,658
Past due 90 days or Greater and Still Accruing	3,104	289
Loans, current	1,948,743	1,930,412
Total loans	1,957,672	1,943,070
Commercial Real Estate: Farmland & Commercial \| 30 to 59 Days
Financing receivable recorded investment
Past due	1,406	3,454
Commercial Real Estate: Farmland & Commercial \| 60 to 89 Days
Financing receivable recorded investment
Past due	1,290	3,054
Commercial Real Estate: Farmland & Commercial \| 90 Days or Greater
Financing receivable recorded investment
Past due	6,233	6,150
Commercial Real Estate: Multifamily
Financing receivable recorded investment
Past due	532	596
Loans, current	144,566	139,870
Total loans	145,098	140,466
Commercial Real Estate: Multifamily \| 30 to 59 Days
Financing receivable recorded investment
Past due		44
Commercial Real Estate: Multifamily \| 90 Days or Greater
Financing receivable recorded investment
Past due	532	552
Residential First Lien
Financing receivable recorded investment
Past due	8,570	11,108
Past due 90 days or Greater and Still Accruing	4,296	3,756
Loans, current	409,964	410,796
Total loans	418,534	421,904
Residential First Lien \| 30 to 59 Days
Financing receivable recorded investment
Past due	2,960	5,615
Residential First Lien \| 60 to 89 Days
Financing receivable recorded investment
Past due	933	1,350
Residential First Lien \| 90 Days or Greater
Financing receivable recorded investment
Past due	4,677	4,143
Residential Junior Lien
Financing receivable recorded investment
Past due	1,269	1,480
Past due 90 days or Greater and Still Accruing	343	382
Loans, current	627,212	608,838
Total loans	628,481	610,318
Residential Junior Lien \| 30 to 59 Days
Financing receivable recorded investment
Past due	739	762
Residential Junior Lien \| 60 to 89 Days
Financing receivable recorded investment
Past due	30	178
Residential Junior Lien \| 90 Days or Greater
Financing receivable recorded investment
Past due	500	540
Consumer
Financing receivable recorded investment
Past due	977	1,418
Past due 90 days or Greater and Still Accruing	329	387
Loans, current	52,637	53,750
Total loans	53,614	55,168
Consumer \| 30 to 59 Days
Financing receivable recorded investment
Past due	516	910
Consumer \| 60 to 89 Days
Financing receivable recorded investment
Past due	94	95
Consumer \| 90 Days or Greater
Financing receivable recorded investment
Past due	367	413
Foreign
Financing receivable recorded investment
Past due	3,982	1,753
Past due 90 days or Greater and Still Accruing	122	11
Loans, current	159,647	165,467
Total loans	163,629	167,220
Foreign \| 30 to 59 Days
Financing receivable recorded investment
Past due	2,832	931
Foreign \| 60 to 89 Days
Financing receivable recorded investment
Past due	638	425
Foreign \| 90 Days or Greater
Financing receivable recorded investment
Past due	$ 512	$ 397

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Financing receivables that are current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524

+ Details

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

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5074-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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: