Junior Subordinated Deferrable Interest Debentures (Details)	9 Months Ended
Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Junior Subordinated Interest Deferrable Debentures
Number of statutory business trusts issuing trust preferred securities \| item	6
Junior subordinated deferrable interest debentures \| $	$ 160,416,000	$ 160,416,000
Maximum number of consecutive quarterly period available for deferral of interest payment on Trusts VI, VIII, IX, X, XI and XII \| item	20
Percentage of capital securities issued by trust qualifying as Tier I capital, maximum	25.00%
Percentage of capital securities issued by trust qualifying as Tier II capital, minimum	25.00%
Capital securities issued by the trust, qualifying as Tier I capital \| $	$ 160,416,000	$ 160,416,000

Represents capital securities issued by trust that is qualified as Tier I capital as on the reporting date.

No definition available.

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Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the maximum number of consecutive quarterly periods available to the entity for deferring the payment of interest on Trusts VI, VII, VIII, IX, X, XI and XII.

+ References

No definition available.

+ Details

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Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
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Period Type:	duration

- Definition

Represents the maximum percentage of capital securities issued by the trust qualifying as Tier I capital on an aggregate basis.

+ References

No definition available.

+ Details

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Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of capital securities issued by the trust qualifying as Tier II capital on an aggregate basis.

+ References

No definition available.

+ Details

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Namespace Prefix:	iboc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of statutory business trusts formed for the purpose of issuing trust preferred securities.

+ References

No definition available.

+ Details

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Namespace Prefix:	iboc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- References

No definition available.

+ Details

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