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Investment Securities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Held-to-maturity securities    
Amortized cost $ 2,400us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment $ 2,400us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Estimated fair value 2,400us-gaap_HeldToMaturitySecuritiesFairValue 2,400us-gaap_HeldToMaturitySecuritiesFairValue
Carrying value 2,400us-gaap_HeldToMaturitySecurities 2,400us-gaap_HeldToMaturitySecurities
Available-for-sale securities    
Amortized cost 4,914,428us-gaap_AvailableForSaleSecuritiesAmortizedCost 5,372,594us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross unrealized gains 68,516iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments 65,104iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Gross unrealized losses (50,981)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments (133,119)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Total investment securities 4,931,963us-gaap_AvailableForSaleSecurities 5,304,579us-gaap_AvailableForSaleSecurities
Assets/Liabilities Measured at Fair Value    
Available-for-sale securities    
Total investment securities 4,931,963us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
5,304,579us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Carrying value    
Available-for-sale securities    
Total investment securities 4,931,963us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
5,304,579us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Other securities    
Held-to-maturity securities    
Amortized cost 2,400us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
2,400us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Estimated fair value 2,400us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
2,400us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Carrying value 2,400us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
2,400us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Available-for-sale securities    
Amortized cost 20,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
 
Other securities | Assets/Liabilities Measured at Fair Value    
Available-for-sale securities    
Total investment securities 20,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
 
Other securities | Carrying value    
Available-for-sale securities    
Total investment securities 20,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
 
Residential mortgage-backed securities    
Available-for-sale securities    
Amortized cost 4,597,590us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
5,096,165us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross unrealized gains 47,960iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
56,110iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses (45,178)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
(124,574)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Residential mortgage-backed securities | Assets/Liabilities Measured at Fair Value    
Available-for-sale securities    
Total investment securities 4,600,372us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
5,027,701us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Residential mortgage-backed securities | Carrying value    
Available-for-sale securities    
Total investment securities 4,600,372us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
5,027,701us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Mortgage-backed securities by Ginnie Mae    
Available-for-sale securities    
Total investment securities 1,503,774us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
1,799,807us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Mortgage-backed securities by Fannie Mae and Freddie Mac    
Available-for-sale securities    
Total investment securities 3,072,535us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
3,200,042us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Mortgage-backed securities by non-government entities    
Available-for-sale securities    
Total investment securities 24,063us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
27,852us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
States and political subdivisions    
Available-for-sale securities    
Amortized cost 268,763us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
248,354us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized gains 19,131iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
8,063iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized losses (5,618)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(8,007)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
States and political subdivisions | Assets/Liabilities Measured at Fair Value    
Available-for-sale securities    
Total investment securities 282,276us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
248,410us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
States and political subdivisions | Carrying value    
Available-for-sale securities    
Total investment securities 282,276us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
248,410us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Other    
Available-for-sale securities    
Amortized cost 28,075us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
28,075us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Gross unrealized gains 1,425iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
931iboc_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Gross unrealized losses (185)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(538)iboc_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Other | Assets/Liabilities Measured at Fair Value    
Available-for-sale securities    
Total investment securities 29,315us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
28,468us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Other | Carrying value    
Available-for-sale securities    
Total investment securities $ 29,315us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
$ 28,468us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember