Capital Requirements (Details) - USD ($)			12 Months Ended
Sep. 19, 2018	Dec. 23, 2008	Dec. 31, 2018
Capital Requirements
Warrants to purchase entity's common stock (in shares)		1,326,238
Warrants, exercise price (in dollars per share)		$ 24.43
Warrants, term		10 years
Maximum cash dividend paid in semi-annual period (in dollars per share)	$ 0.33
Payments for repurchase of warrants	$ 29,005,000		$ 29,005,000
Dividend payable by subsidiaries, maximum			$ 768,900,000

Period representing the term of warrant.

No definition available.

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- 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

- References

No definition available.

+ Details

Name:	us-gaap_BankingAndThriftAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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
Balance Type:	na
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The aggregate amount paid by the entity to reacquire the right to purchase equity shares at a predetermined price, usually issued together with corporate debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration