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Junior Subordinated Deferrable Interest Debentures (Details) (USD $) | 12 Months Ended | 0 Months Ended |
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Dec. 31, 2014 item | Feb. 11, 2014 | Dec. 31, 2013
Junior subordinated deferrable interest debentures, major types of business trusts |  |  |
Number of statutory business trusts issuing trust preferred securities | 7 iboc_NumberOfStatutoryBusinessTrustsIssuingPreferredSecurities |  |
Junior subordinated deferrable interest debentures | $ 175,416,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust |  | $ 190,726,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Maximum number of consecutive quarterly period available for deferral of interest payment on Trusts VI, VII, VIII, IX, X, XI and XII | 20 iboc_MaximumNumberOfConsecutiveQuarterlyPeriodsAvailableForDeferralOfPaymentOfInterest |  |
Percentage of capital securities issued by trust qualifying as Tier I capital, maximum | 25.00%iboc_MaximumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital |  |
Percentage of capital securities issued by trust qualifying as Tier II capital, minimum | 25.00%iboc_MinimumPercentageOfCapitalSecuritiesIssuedByTrustQualifyingAsTierTwoCapital |  |
Capital securities issued by the trust, qualifying as Tier I capital | 175,416,000 iboc_CapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital |  | 190,726,000 iboc_CapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Trust VII |  |  |
Junior subordinated deferrable interest debentures, major types of business trusts |  |  |
Redemption price discount (as a percent) |  | 6.00%iboc_DebtInstrumentsRedemptionPriceDiscount / us-gaap_DebtInstrumentAxis = iboc_TrustSevenMember |
Retirement of capital securities |  | 10,310,000 us-gaap_RepaymentsOfDebt / us-gaap_DebtInstrumentAxis = iboc_TrustSevenMember |
Trust XI |  |  |
Junior subordinated deferrable interest debentures, major types of business trusts |  |  |
Redemption price discount (as a percent) | 23.60%iboc_DebtInstrumentsRedemptionPriceDiscount / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember |  |
Retirement of capital securities | 5,000,000 us-gaap_RepaymentsOfDebt / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember |  |
Junior subordinated deferrable interest debentures | $ 27,990,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / us-gaap_DebtInstrumentAxis = iboc_TrustElevenMember |  |

X
- Definition Represents capital securities issued by trust that is qualified as Tier I capital as on the reporting date. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the percentage of discount from redemption price of debt. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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
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X
- 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
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Namespace Prefix: | iboc_
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X
- 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
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Namespace Prefix: | iboc_
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X
- Definition Number of statutory business trusts formed for the purpose of issuing trust preferred securities. + References No definition available. + Details
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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 + Details
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- 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
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- Details
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