Allowance for Probable Loan Losses (Details 5) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing receivable recorded investment
Past due 30-59 days	$ 17,261,000	$ 19,891,000
Past due 60-89 days	12,215,000	5,330,000
Past due 90 days or greater	67,770,000	60,537,000
Past due 90 days or greater and still accruing	9,988,000	7,197,000	15,033,000
Past due, total	97,246,000	85,758,000
Loans, current	5,581,999,000	5,113,477,000
Total loans	5,679,245,000	5,199,235,000
Commercial
Financing receivable recorded investment
Past due 30-59 days	4,103,000	4,240,000
Past due 60-89 days	2,665,000	538,000
Past due 90 days or greater	40,665,000	36,066,000
Past due 90 days or greater and still accruing	2,890,000	2,051,000
Past due, total	47,433,000	40,844,000
Loans, current	1,042,053,000	1,001,798,000
Total loans	1,089,486,000	1,042,642,000
Commercial real estate: other construction and land development
Financing receivable recorded investment
Past due 30-59 days	596,000	1,042,000
Past due 60-89 days	10,000
Past due 90 days or greater	8,707,000	9,942,000
Past due 90 days or greater and still accruing	439,000	62,000
Past due, total	9,313,000	10,984,000
Loans, current	1,405,664,000	1,197,524,000
Total loans	1,414,977,000	1,208,508,000
Commercial real estate: farmland and commercial
Financing receivable recorded investment
Past due 30-59 days	2,905,000	6,216,000
Past due 60-89 days	7,131,000	520,000
Past due 90 days or greater	10,724,000	6,990,000
Past due 90 days or greater and still accruing	1,711,000	417,000
Past due, total	20,760,000	13,726,000
Loans, current	1,880,639,000	1,736,313,000
Total loans	1,901,399,000	1,750,039,000
Commercial real estate: multifamily
Financing receivable recorded investment
Past due 30-59 days	351,000	39,000
Past due 60-89 days		142,000
Past due 90 days or greater	856,000	295,000
Past due 90 days or greater and still accruing	21,000
Past due, total	1,207,000	476,000
Loans, current	115,492,000	101,622,000
Total loans	116,699,000	102,098,000
Residential: first lien
Financing receivable recorded investment
Past due 30-59 days	5,895,000	4,758,000
Past due 60-89 days	1,864,000	3,046,000
Past due 90 days or greater	4,267,000	4,541,000
Past due 90 days or greater and still accruing	3,901,000	3,518,000
Past due, total	12,026,000	12,345,000
Loans, current	410,000,000	426,117,000
Total loans	422,026,000	438,462,000
Residential: junior lien
Financing receivable recorded investment
Past due 30-59 days	899,000	606,000
Past due 60-89 days	231,000	198,000
Past due 90 days or greater	1,931,000	1,900,000
Past due 90 days or greater and still accruing	431,000	368,000
Past due, total	3,061,000	2,704,000
Loans, current	485,239,000	406,526,000
Total loans	488,300,000	409,230,000
Consumer Loan
Financing receivable recorded investment
Past due 30-59 days	896,000	1,523,000
Past due 60-89 days	216,000	469,000
Past due 90 days or greater	507,000	803,000
Past due 90 days or greater and still accruing	482,000	781,000
Past due, total	1,619,000	2,795,000
Loans, current	59,518,000	63,619,000
Total loans	61,137,000	66,414,000
Foreign
Financing receivable recorded investment
Past due 30-59 days	1,616,000	1,467,000
Past due 60-89 days	98,000	417,000
Past due 90 days or greater	113,000
Past due 90 days or greater and still accruing	113,000
Past due, total	1,827,000	1,884,000
Loans, current	183,394,000	179,958,000
Total loans	$ 185,221,000	$ 181,842,000

Financing receivables that are less than 60 days past due but more than 29 days past due.

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=35738203&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivables that are less than 90 days past due but more than 59 days past due.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: 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=35738203&loc=d3e5111-111524

+ Details

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

- 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

+ Details

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

- Definition

Financing receivables that are equal to or greater than 90 days past due.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
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

+ 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 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

Reference 2: 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=28367877&loc=d3e4428-111522

+ Details

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

- 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_CommercialRealEstateOtherConstructionAndLandDevelopmentReceivableMember
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:

- 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_CommercialLoanMember
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: