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Summary of Significant Accounting Policies (Details) (USD $) | 0 Months Ended | 12 Months Ended
--- | --- | ---
Oct. 01, 2014 | Dec. 31, 2014 item | Dec. 31, 2013 | Dec. 31, 2012
Summary of Significant Accounting Policies |  |  |  |
Number of insurance-related subsidiaries |  | 2 iboc_EntityNumberOfInsuranceRelatedSubsidiaries |  |
Investment Securities |  |  |  |
Number of components in which other-than-temporary impairment is segregated |  | 2 iboc_NumberOfComponentsOtherThanTemporaryImpairmentSegregated |  |
Non-Accrual Loans |  |  |  |
Period of charge off for past due unsecured consumer loans |  | 90 days |  |
Minimum period that past due unsecured loans outstanding may not be placed on nonaccrual status under special circumstances |  | 90 days |  |
Maximum period of non-consumer loans outstanding that is used to evaluate whether loans should be placed on non-accrual status |  | 90 days |  |
Other Real Estate Owned |  |  |  |
Other real estate owned |  | $ 69,872,000 iboc_OtherRealEstateAndForeclosedAssetsInUse | $ 85,879,000 iboc_OtherRealEstateAndForeclosedAssetsInUse |
Income Taxes |  |  |  |
Percentage of likelihood of realization of recognized tax benefit |  | 50.00%iboc_PercentageOfLikelihoodOfRealizationOfRecognizedTaxBenefit |  |
Liability for unrecognized tax benefits |  | 0 us-gaap_UnrecognizedTaxBenefits | 0 us-gaap_UnrecognizedTaxBenefits |
Interest expense related to uncertain tax positions |  | 0 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | 0 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | 0 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense
Penalties related to uncertain tax positions |  | 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense | 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense | 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense
Goodwill and Identified Intangible Assets |  |  |  |
Goodwill impairment loss | 0 us-gaap_GoodwillImpairmentLoss |  |  |
Impairment of identified intangible assets |  | $ 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill |  |
Segments of an Enterprise and Related Information |  |  |  |
Number of operating segments |  | 1 us-gaap_NumberOfOperatingSegments |  |
Number of active operating bank subsidiaries |  | 4 iboc_NumberOfActiveOperatingSubsidiaries |  |

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- Definition Represents the maximum period of non-consumer loans outstanding that is used to evaluate whether loans should be placed on non-accrual status. + References No definition available. + Details
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- Definition Represents the minimum period that past due unsecured loans outstanding may not be placed on nonaccrual status under special circumstances. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings. + References No definition available. + Details
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- Definition Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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