Represents capital securities issued by trust that is qualified as Tier I capital as on the reporting date.

No definition available.

Name:	iboc_CapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the percentage of discount from redemption price of debt.

+ References

No definition available.

+ Details

Name:	iboc_DebtInstrumentsRedemptionPriceDiscount
Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum number of consecutive quarterly periods available to the entity for deferring the payment of interest on Trusts VI, VII, VIII, IX, X, XI and XII.

+ References

No definition available.

+ Details

Name:	iboc_MaximumNumberOfConsecutiveQuarterlyPeriodsAvailableForDeferralOfPaymentOfInterest
Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum percentage of capital securities issued by the trust qualifying as Tier I capital on an aggregate basis.

+ References

No definition available.

+ Details

Name:	iboc_MaximumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of capital securities issued by the trust qualifying as Tier II capital on an aggregate basis.

+ References

No definition available.

+ Details

Name:	iboc_MinimumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierTwoCapital
Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of statutory business trusts formed for the purpose of issuing trust preferred securities.

+ References

No definition available.

+ Details

Name:	iboc_NumberOfStatutoryBusinessTrustsIssuingPreferredSecurities
Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=iboc_TrustElevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=iboc_TrustTenAndElevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=iboc_TrustTenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Junior Subordinated Deferrable Interest Debentures (Details)		9 Months Ended
	Jul. 29, 2015 USD ($)	Sep. 30, 2016 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Junior subordinated deferrable interest debentures, major types of business trusts
Number of statutory business trusts issuing trust preferred securities \| item		6
Junior subordinated deferrable interest debentures		$ 161,416,000		$ 161,416,000
Maximum number of consecutive quarterly period available for deferral of interest payment on Trusts VI, VII, VIII, IX, X, XI and XII \| item		20
Percentage of capital securities issued by trust qualifying as Tier I capital, maximum		25.00%
Percentage of capital securities issued by trust qualifying as Tier II capital, minimum		25.00%
Capital securities issued by the trust, qualifying as Tier I capital		$ 161,416,000	$ 161,416,000	$ 161,416,000
Trust XI
Junior subordinated deferrable interest debentures, major types of business trusts
Retirement of capital securities	$ 1,000,000
Junior subordinated deferrable interest debentures	$ 27,900,000	26,990,000
Trust X and XI
Junior subordinated deferrable interest debentures, major types of business trusts
Redemption price discount (as a percent)	24.50%
Trust X
Junior subordinated deferrable interest debentures, major types of business trusts
Retirement of capital securities	$ 13,000,000
Junior subordinated deferrable interest debentures	$ 34,021,000	$ 21,021,000