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Capital Requirements (Details) - USD ($) |  | 12 Months Ended
--- | --- | ---
Dec. 23, 2008 | Dec. 31, 2017
Capital Requirements |  |
Warrants to purchase entity's common stock (in shares) | 1,326,238 |
Warrants, exercise price (in dollars per share) | $ 24.43 | $ 24.43
Maximum cash dividend paid in semi-annual period (in dollars per share) |  | $ 0.33
Warrants, term | 10 years |
Dividend payable by subsidiaries, maximum |  | $ 731,850,000

X
- Definition Period representing the term of warrant. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the maximum amount of dividend the subsidiary banks could pay to the entity. + References No definition available. + Details
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Namespace Prefix: | iboc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the maximum amount of cash dividends paid per semi-annual period before adjustments will be made to the exercise price of the warrant. + References No definition available. + Details
Name: | iboc_SemiAnnualCashDividendsPaidMaximum
Namespace Prefix: | iboc_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Definition Exercise price per share or per unit of warrants or rights outstanding. + 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.(i)(4)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
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Period Type: | instant

X
- Definition Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares. + 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.(i)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant