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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   2iboc_EntityNumberOfInsuranceRelatedSubsidiaries    
Investment Securities        
Number of components in which other-than-temporary impairment is segregated   2iboc_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   $ 69,872,000iboc_OtherRealEstateAndForeclosedAssetsInUse $ 85,879,000iboc_OtherRealEstateAndForeclosedAssetsInUse  
Income Taxes        
Percentage of likelihood of realization of recognized tax benefit   50.00%iboc_PercentageOfLikelihoodOfRealizationOfRecognizedTaxBenefit    
Liability for unrecognized tax benefits   0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits  
Interest expense related to uncertain tax positions   0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense 0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense 0us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense
Penalties related to uncertain tax positions   0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense
Goodwill and Identified Intangible Assets        
Goodwill impairment loss 0us-gaap_GoodwillImpairmentLoss      
Impairment of identified intangible assets   $ 0us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill    
Segments of an Enterprise and Related Information        
Number of operating segments   1us-gaap_NumberOfOperatingSegments    
Number of active operating bank subsidiaries   4iboc_NumberOfActiveOperatingSubsidiaries