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Securities Sold Under Repurchase Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Securities Sold Under Repurchase Agreements
Average outstanding amount	$ 893,830,000iboc_SecuritiesSoldUnderAgreementsToRepurchaseAverageOutstandingAmount	$ 1,041,186,000iboc_SecuritiesSoldUnderAgreementsToRepurchaseAverageOutstandingAmount
Maximum amount outstanding at any month end	892,341,000iboc_SecuritiesSoldUnderAgreementsToRepurchaseMaximumOutstandingAmountAtAnyMonthEnd	1,119,531,000iboc_SecuritiesSoldUnderAgreementsToRepurchaseMaximumOutstandingAmountAtAnyMonthEnd
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	1,127,170,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts	1,313,637,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts
Collateralized securities, Fair Value of Securities Sold	1,134,418,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue	1,317,998,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue
Repurchase Borrowing, Balance of Liability	858,350,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase	957,381,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	2.79%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate	2.91%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Overnight agreements
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	366,731,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember	332,042,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember
Collateralized securities, Fair Value of Securities Sold	370,704,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember	341,238,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember
Repurchase Borrowing, Balance of Liability	236,077,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember	209,899,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.16%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember	0.17%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOvernightMember
1 to 29 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	3,717,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember	27,954,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember
Collateralized securities, Fair Value of Securities Sold	3,781,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember	28,288,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember
Repurchase Borrowing, Balance of Liability	1,016,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember	18,666,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.45%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember	0.40%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityUpTo30DaysMember
30 to 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	13,399,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember	8,106,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember
Collateralized securities, Fair Value of Securities Sold	13,628,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember	8,263,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember
Repurchase Borrowing, Balance of Liability	6,705,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember	3,295,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.38%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember	0.43%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_Maturity30To90DaysMember
Over 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	743,323,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember	945,535,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember
Collateralized securities, Fair Value of Securities Sold	746,305,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember	940,209,000us-gaap_AssetsSoldUnderAgreementsToRepurchaseMarketValue / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember
Repurchase Borrowing, Balance of Liability	$ 614,552,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember	$ 725,521,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.83%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember	3.78%us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate / us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis = us-gaap_MaturityOver90DaysMember

X

- Definition

Represents the average amount of securities sold under repurchase agreements during the period.

+ References

No definition available.

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- Definition

Represents the maximum balance outstanding at any month end during the period as of the balance sheet date.

+ References

No definition available.

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- Definition

Carrying amount of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))

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Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

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- Definition

Market value of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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