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Income Taxes (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes
Income tax expense differences from the amount computed by applying the U.S. Federal income tax rate to income before income taxes (as a percent)	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Reasons for the difference of income tax expense
Computed expected tax expense	$ 80,560us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 64,183us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 55,634us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Change in taxes resulting from:
Tax-exempt interest income	(4,554)us-gaap_IncomeTaxReconciliationTaxExemptIncome	(4,828)us-gaap_IncomeTaxReconciliationTaxExemptIncome	(4,381)us-gaap_IncomeTaxReconciliationTaxExemptIncome
State tax, net of federal income taxes and tax credit	3,377us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	(110)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	1,446us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Tax refunds		(966)us-gaap_IncomeTaxReconciliationDeductions
Other investment income	(3,615)iboc_IncomeTaxReconciliationNoninterestIncomeOther	(2,656)iboc_IncomeTaxReconciliationNoninterestIncomeOther	(2,691)iboc_IncomeTaxReconciliationNoninterestIncomeOther
Other	1,019us-gaap_IncomeTaxReconciliationOtherAdjustments	616us-gaap_IncomeTaxReconciliationOtherAdjustments	557us-gaap_IncomeTaxReconciliationOtherAdjustments
Total income taxes	$ 76,787us-gaap_IncomeTaxExpenseBenefit	$ 56,239us-gaap_IncomeTaxExpenseBenefit	$ 50,565us-gaap_IncomeTaxExpenseBenefit

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The portion of the difference between total income tax expense or benefit as reported in the Income Statement for the current period and the expected income tax expense or benefit computed attributable to other investment income.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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