Represents the maximum fixed expiration date upon which commitments are set to expire.

No definition available.

Name:	iboc_CreditRiskCommitmentsExpirationDate
Namespace Prefix:	iboc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

+ Details

Name:	us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=iboc_CommercialLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=iboc_UnsecuredLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments with Off-Statement of Condition Risk and Concentrations of Credit Risk (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Maximum [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Credit risk commitment, maximum expiration date	1 year
Commitments to Extend Credit [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Outstanding amount of financial instruments whose, contract amounts represent credit risks	$ 2,354,010,000
Line of Credit [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Outstanding amount of financial instruments whose, contract amounts represent credit risks	13,579,000
Standby Letters of Credit [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Outstanding amount of financial instruments whose, contract amounts represent credit risks	118,381,000
Guarantee obligations, maximum exposure	118,381,000
Commercial Letters of Credit [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Outstanding amount of financial instruments whose, contract amounts represent credit risks	341,000
Unsecured Letters of Credit [Member]
Financial amounts of instruments, whose contract amounts represent credit risks
Unsecured letters of credit	$ 39,487,000	$ 49,965,000