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Deposits (Details 2) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Scheduled maturities of time deposits    
2015 $ 2,177,266,000us-gaap_TimeDepositMaturitiesYearOne  
2016 203,928,000us-gaap_TimeDepositMaturitiesYearTwo  
2017 76,206,000us-gaap_TimeDepositMaturitiesYearThree  
2018 23,481,000us-gaap_TimeDepositMaturitiesYearFour  
2019 1,751,000us-gaap_TimeDepositMaturitiesYearFive  
Thereafter 60,000us-gaap_TimeDepositMaturitiesAfterYearFive  
Total time, certificates of deposit 2,482,692,000us-gaap_TimeDeposits 2,651,303,000us-gaap_TimeDeposits
Time Deposits 250,000 or exceed 1,027,000iboc_TimeDeposits250000OrExceedFdicInsurance 1,076,000iboc_TimeDeposits250000OrExceedFdicInsurance
Scheduled maturities of time deposits in amounts of $100,000 or more    
Due within 3 months or less 631,083,000us-gaap_ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsOrLess  
Due after 3 months and within 6 months 409,400,000us-gaap_ContractualMaturitiesTimeDeposits100000OrMoreThreeMonthsThroughSixMonths  
Due after 6 months and within 12 months 443,860,000us-gaap_ContractualMaturitiesTimeDeposits100000OrMoreSixMonthsThrough12Months  
Due after 12 months 215,795,000us-gaap_ContractualMaturitiesTimeDeposits100000OrMoreAfter12Months  
Total $ 1,700,138,000us-gaap_TimeDeposits100000OrMore