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Other Borrowed Funds (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Dec. 31, 2014
Other Borrowed Funds |  |
Other borrowed funds | $ 941,457 us-gaap_AdvancesFromFederalHomeLoanBanks | $ 1,073,944 us-gaap_AdvancesFromFederalHomeLoanBanks
Percentage of decrease in other borrowed funds | 12.30%iboc_PercentageOfAdvancesFromFederalHomeLoanBanksIncreaseDecrease |

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- Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage change in other borrowed funds. Borrowed funds primarily consist of short and long term borrowings from the Federal Home Loan Bank. + References No definition available. + Details
Name: | iboc_PercentageOfAdvancesFromFederalHomeLoanBanksIncreaseDecrease
Namespace Prefix: | iboc_
Data Type: | num:percentItemType
Balance Type: | na
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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
Name: | us-gaap_AdvancesFromFederalHomeLoanBanks
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant