Allowance for Probable Loan Losses (Aging By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing receivable recorded investment
Past due	$ 59,635	$ 68,273
Past due 90 days or Greater and Still Accruing	8,392	11,616
Loans, current	5,827,027	5,882,641
Total loans	5,886,662	5,950,914
30 to 59 Days
Financing receivable recorded investment
Past due	21,807	15,584
60 to 89 Days
Financing receivable recorded investment
Past due	5,820	5,219
90 Days or Greater
Financing receivable recorded investment
Past due	32,008	47,470
Commercial
Financing receivable recorded investment
Past due	24,832	32,916
Past due 90 days or Greater and Still Accruing	2,587	2,566
Loans, current	888,987	933,936
Total loans	913,819	966,852
Commercial \| 30 to 59 Days
Financing receivable recorded investment
Past due	6,372	3,361
Commercial \| 60 to 89 Days
Financing receivable recorded investment
Past due	1,418	940
Commercial \| 90 Days or Greater
Financing receivable recorded investment
Past due	17,042	28,615
Commercial Real Estate: Other Construction and Land Development
Financing receivable recorded investment
Past due	4,235	3,988
Past due 90 days or Greater and Still Accruing	12
Loans, current	1,606,841	1,645,839
Total loans	1,611,076	1,649,827
Commercial Real Estate: Other Construction and Land Development \| 30 to 59 Days
Financing receivable recorded investment
Past due	2,366	193
Commercial Real Estate: Other Construction and Land Development \| 60 to 89 Days
Financing receivable recorded investment
Past due		293
Commercial Real Estate: Other Construction and Land Development \| 90 Days or Greater
Financing receivable recorded investment
Past due	1,869	3,502
Commercial Real Estate: Farmland & Commercial
Financing receivable recorded investment
Past due	14,136	12,304
Past due 90 days or Greater and Still Accruing	1,341	3,373
Loans, current	1,989,727	1,983,144
Total loans	2,003,863	1,995,448
Commercial Real Estate: Farmland & Commercial \| 30 to 59 Days
Financing receivable recorded investment
Past due	5,728	2,684
Commercial Real Estate: Farmland & Commercial \| 60 to 89 Days
Financing receivable recorded investment
Past due	1,314	1,328
Commercial Real Estate: Farmland & Commercial \| 90 Days or Greater
Financing receivable recorded investment
Past due	7,094	8,292
Commercial Real Estate: Multifamily
Financing receivable recorded investment
Past due	957	1,317
Past due 90 days or Greater and Still Accruing		49
Loans, current	117,322	138,131
Total loans	118,279	139,448
Commercial Real Estate: Multifamily \| 30 to 59 Days
Financing receivable recorded investment
Past due	194	49
Commercial Real Estate: Multifamily \| 60 to 89 Days
Financing receivable recorded investment
Past due	192	442
Commercial Real Estate: Multifamily \| 90 Days or Greater
Financing receivable recorded investment
Past due	571	826
Residential First Lien
Financing receivable recorded investment
Past due	9,760	11,139
Past due 90 days or Greater and Still Accruing	3,428	4,093
Loans, current	407,922	399,105
Total loans	417,682	410,244
Residential First Lien \| 30 to 59 Days
Financing receivable recorded investment
Past due	3,748	5,299
Residential First Lien \| 60 to 89 Days
Financing receivable recorded investment
Past due	2,160	1,545
Residential First Lien \| 90 Days or Greater
Financing receivable recorded investment
Past due	3,852	4,295
Residential Junior Lien
Financing receivable recorded investment
Past due	1,770	1,772
Past due 90 days or Greater and Still Accruing	504	640
Loans, current	594,069	550,566
Total loans	595,839	552,338
Residential Junior Lien \| 30 to 59 Days
Financing receivable recorded investment
Past due	786	713
Residential Junior Lien \| 60 to 89 Days
Financing receivable recorded investment
Past due	312	413
Residential Junior Lien \| 90 Days or Greater
Financing receivable recorded investment
Past due	672	646
Consumer
Financing receivable recorded investment
Past due	1,303	1,308
Past due 90 days or Greater and Still Accruing	332	453
Loans, current	54,693	56,436
Total loans	55,996	57,744
Consumer \| 30 to 59 Days
Financing receivable recorded investment
Past due	692	646
Consumer \| 60 to 89 Days
Financing receivable recorded investment
Past due	277	175
Consumer \| 90 Days or Greater
Financing receivable recorded investment
Past due	334	487
Foreign
Financing receivable recorded investment
Past due	2,642	3,529
Past due 90 days or Greater and Still Accruing	188	442
Loans, current	167,466	175,484
Total loans	170,108	179,013
Foreign \| 30 to 59 Days
Financing receivable recorded investment
Past due	1,921	2,639
Foreign \| 60 to 89 Days
Financing receivable recorded investment
Past due	147	83
Foreign \| 90 Days or Greater
Financing receivable recorded investment
Past due	$ 574	$ 807

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: