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Capital Ratios (Details) (USD $)	0 Months Ended	3 Months Ended
	Jan. 01, 2015	Mar. 31, 2015	Dec. 31, 2014	Jul. 31, 2013
Common Equity Tier 1 ("CET1") ratio		15.60%iboc_CommonEquityTierOneRatio
Tier 1 Capital (to Average Assets), Actual Ratio (as a percent)		12.27%us-gaap_TierOneLeverageCapitalToAverageAssets	12.33%us-gaap_TierOneLeverageCapitalToAverageAssets
Tier 1 Capital (to Risk Weighted Assets), Actual Ratio (as a percent)		17.83%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets	19.34%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Total Capital (to Risk Weighted Assets), Actual Ratio (as a percent)	4.50%us-gaap_CapitalToRiskWeightedAssets	18.67%us-gaap_CapitalToRiskWeightedAssets	20.24%us-gaap_CapitalToRiskWeightedAssets
Total Capital (to Risk Weighted Assets), For Capital Adequacy Purposes Ratio (as a percent)	2.50%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Term of mandatory compliance	4 years
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		$ 175,416,000iboc_CapitalSecuritiesIssuedByTrustQualifyingAsTierOneCapital
Minimum
Common Equity Tier 1 ("CET1") ratio		7.00%iboc_CommonEquityTierOneRatio / us-gaap_RangeAxis = us-gaap_MinimumMember
Tier 1 Capital (to Risk Weighted Assets), Actual Ratio (as a percent)				4.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / us-gaap_RangeAxis = us-gaap_MinimumMember
Forecast
Total Capital (to Risk Weighted Assets), For Capital Adequacy Purposes Ratio (as a percent)		0.625%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember

X

- Definition

Represents capital securities issued by trust that is qualified as Tier I capital as on the reporting date.

+ References

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Represents information pertaining to Common Equity Tier 1 ("CET1")ratio.

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- Definition

Period representing the new rule of mandatory compliance.

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- 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.

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- Definition

Represents the minimum percentage of capital securities issued by the trust qualifying as Tier II capital on an aggregate basis.

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No definition available.

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- Definition

The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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