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Other Borrowed Funds (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Federal Home Loan Bank advances |  |
Balance at year end | $ 1,073,944,000 us-gaap_AdvancesFromFederalHomeLoanBanks | $ 1,223,950,000 us-gaap_AdvancesFromFederalHomeLoanBanks
Federal Home Loan Bank advances short-term |  |
Federal Home Loan Bank advances |  |
Balance at year end | 1,067,700,000 us-gaap_AdvancesFromFederalHomeLoanBanks / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember | 1,215,000,000 us-gaap_AdvancesFromFederalHomeLoanBanks / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember
Rate on balance outstanding at year end (as a percent) | 0.15%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember | 0.17%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember
Average daily balance | 1,077,973,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember | 833,868,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember
Average rate (as a percent) | 0.16%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateForYear / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember | 0.16%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateForYear / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember
Maximum amount outstanding at any month end | 1,352,500,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember | 1,280,500,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember
Federal Home Loan Bank advances long-term |  |
Federal Home Loan Bank advances |  |
Balance at year end | 6,244,000 us-gaap_AdvancesFromFederalHomeLoanBanks / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 8,950,000 us-gaap_AdvancesFromFederalHomeLoanBanks / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Rate on balance outstanding at year end (as a percent) | 3.51%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 3.53%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Average daily balance | 7,338,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 7,289,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageBalanceOfAgreementsOutstanding / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Average rate (as a percent) | 3.51%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateForYear / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 3.53%us-gaap_FederalHomeLoanBankAdvancesActivityForYearAverageInterestRateForYear / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Maximum amount outstanding at any month end | 8,934,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember | 8,993,000 us-gaap_FederalHomeLoanBankAdvancesActivityForYearMaximumOutstandingAtAnyMonthEnd / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember
Amortization of long-term advances for the next five years | $ 160,000 iboc_FederalHomeLoanBankAdvancesActivityForNextFiveYearsAmortization / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember |
Period of amortization of long-term advances | 5 years |

X
- Definition The annual amortization amount for Federal Home Loan Bank advances for the next five years. Update label as: Annual amortization of long-term advances for the next five years. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This represents the period of amortization of the long-term advances in years. + References No definition available. + Details
Name: | iboc_PeriodOfAmortizationOfLongTermAdvances
Namespace Prefix: | iboc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Details
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Namespace Prefix: | us-gaap_
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- Definition The calculated average balance outstanding for the year for all Federal Home Loan Bank advances as of the Balance Sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | duration

X
- Definition The calculated weighted average interest rate for Federal Home Loan Bank advances outstanding as of the Balance Sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition The calculated weighted average interest rate for Federal Home Loan Bank advances for the period ended as of the Balance Sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The maximum balance outstanding at any month end during the period as of the Balance Sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Period Type: | instant

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- Details
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- Details
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