The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_ThresholdPeriodPastDueForWriteoffOfFinancingReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnallocatedFinancingReceivablesMember
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:

Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Interest Recognized
Total trouble debt restructuring	$ 15,420		$ 15,420		$ 15,719
Period of charge off for past due unsecured commercial loans			90 days
Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	7,549		$ 7,549		8,785
Total impaired loans with no related allowance	30,174		30,174		51,663
Unpaid principal balance:
Total impaired loans with an allowance recorded	10,017		10,017		9,233
Total impaired loans with no related allowance recorded	58,271		58,271		64,788
Related Allowance	1,078		1,078		2,206
Average recorded investment:
Total impaired loans with an allowance recorded	9,376	$ 22,306	9,839	$ 27,126	33,286
Total impaired loans with no related allowance recorded	31,509	39,723	38,093	40,606	42,643
Interest Recognized
Total impaired loans with an allowance recorded	22	23	48	46	92
Total impaired loans with no related allowance recorded	112	105	228	212	106
Commercial
Interest Recognized
Total trouble debt restructuring	2,407		2,407		2,419
Commercial \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	2,319		2,319		4,016
Total impaired loans with no related allowance	10,832		10,832		26,930
Unpaid principal balance:
Total impaired loans with an allowance recorded	3,660		3,660		4,156
Total impaired loans with no related allowance recorded	37,838		37,838		38,845
Related Allowance	831		831		1,704
Average recorded investment:
Total impaired loans with an allowance recorded	3,453	15,121	3,735	17,828	19,313
Total impaired loans with no related allowance recorded	10,837	20,752	15,944	20,288	21,291
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	2	2	1
Commercial Real Estate: Other Construction and Land Development
Interest Recognized
Total trouble debt restructuring	1,879		1,879		2,553
Commercial Real Estate: Other Construction and Land Development \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	161		161		167
Total impaired loans with no related allowance	3,989		3,989		6,054
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		169		169
Total impaired loans with no related allowance recorded	3,995		3,995		6,204
Related Allowance	116		116		100
Average recorded investment:
Total impaired loans with an allowance recorded	162	3,652	163	4,988	7,183
Total impaired loans with no related allowance recorded	5,155	3,291	5,726	3,451	6,045
Interest Recognized
Total impaired loans with no related allowance recorded	20	19	45	37	20
Commercial Real Estate: Farmland & Commercial
Interest Recognized
Total trouble debt restructuring	2,830		2,830		2,853
Commercial Real Estate: Farmland & Commercial \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	5,069		5,069		4,003
Total impaired loans with no related allowance	5,174		5,174		9,803
Unpaid principal balance:
Total impaired loans with an allowance recorded	6,188		6,188		4,309
Total impaired loans with no related allowance recorded	6,144		6,144		10,717
Related Allowance	131		131		202
Average recorded investment:
Total impaired loans with an allowance recorded	5,761	3,533	5,941	4,310	6,790
Total impaired loans with no related allowance recorded	5,709	6,688	6,649	7,778	6,203
Interest Recognized
Total impaired loans with an allowance recorded	22	23	48	46	92
Commercial Real Estate: Multifamily \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance					599
Total impaired loans with no related allowance	590		590		178
Unpaid principal balance:
Total impaired loans with an allowance recorded					599
Total impaired loans with no related allowance recorded	591		591		178
Related Allowance					200
Average recorded investment:
Total impaired loans with no related allowance recorded	301	832	235	839	819
Interest Recognized
Total impaired loans with no related allowance recorded				1
Residential First Lien
Interest Recognized
Total trouble debt restructuring	5,960		5,960		5,316
Residential First Lien \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,683		6,683		5,699
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,758		6,758		5,822
Average recorded investment:
Total impaired loans with no related allowance recorded	6,610	5,289	6,594	5,329	5,315
Interest Recognized
Total impaired loans with no related allowance recorded	74	61	145	123	65
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	839		839		929
Residential Junior Lien \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,021		1,021		950
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,059		1,059		972
Average recorded investment:
Total impaired loans with no related allowance recorded	1,008	1,317	1,022	1,346	1,075
Interest Recognized
Total impaired loans with no related allowance recorded	12	19	27	39	15
Consumer
Interest Recognized
Total trouble debt restructuring	1,132		1,132		1,263
Consumer \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,132		1,132		1,297
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,133		1,133		1,298
Average recorded investment:
Total impaired loans with no related allowance recorded	1,134	1,153	1,172	1,171	1,263
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	1	1	1
Foreign
Interest Recognized
Total trouble debt restructuring	373		373		386
Foreign \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	753		753		752
Unpaid principal balance:
Total impaired loans with no related allowance recorded	753		753		752
Average recorded investment:
Total impaired loans with no related allowance recorded	755	401	751	404	632
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 8	$ 9	$ 4