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Junior Subordinated Interest Deferrable Debentures (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	$ 175,416us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust	$ 175,416us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Trust VI
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	25,774us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustSixMember
Interest rate (as a percent)	3.71%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustSixMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	3.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustSixMember
Trust VIII
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	25,774us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustEightMember
Interest rate (as a percent)	3.30%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustEightMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	3.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustEightMember
Trust IX
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	41,238us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustNineMember
Interest rate (as a percent)	1.88%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustNineMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	1.62%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustNineMember
Trust X
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	34,021us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustTenMember
Interest rate (as a percent)	1.90%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustTenMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	1.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustTenMember
Trust XI
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	27,990us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember
Interest rate (as a percent)	1.88%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	1.62%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember
Trust XII
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	$ 20,619us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustTwelveMember
Interest rate (as a percent)	1.71%us-gaap_DebtInstrumentInterestRateDuringPeriod / us-gaap_DebtInstrumentAxis = iboc_TrustTwelveMember
Interest rate index	LIBOR
Spread on interest rate index (as a percent)	1.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = iboc_TrustTwelveMember

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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- Definition

Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

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-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- 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 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9



Reference 3: 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

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