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Other Borrowed Funds (Details) - USD ($) | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Federal Home Loan Bank advances |  |
Other borrowed funds | $ 674,900,000 | $ 1,195,225,000
Percentage of decrease in other borrowed funds | 43.50% |

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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
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Namespace Prefix: | iboc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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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://fasb.org/us-gaap/role/ref/legacyRef -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

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration