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Other Borrowed Funds (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Federal Home Loan Bank advances
Other borrowed funds	$ 436,280,000	$ 436,327,000
Percentage of increase in other borrowed funds	0.00%

X

- 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_
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Period Type:	duration