Name:	iboc_InterestExpenseNOWAccountsMoneyMarketAccountsAndSavingsDepositDomestic
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The aggregate interest expense incurred on the following foreign deposits in domestic offices: (1) transaction accounts (NOW, and telephone and preauthorized transfer accounts), (2) money market deposits and (3) savings deposit.

No definition available.

Name:	iboc_InterestExpenseNOWAccountsMoneyMarketAccountsAndSavingsDepositForeign
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

No definition available.

Name:	iboc_InterestExpenseTimeDeposits100000OrMoreAbstract
Namespace Prefix:	iboc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Interest expense incurred on domestic time deposits, including certificate of deposits, of 100,000 dollars or more in domestic offices.

No definition available.

Name:	iboc_InterestExpenseTimeDeposits100000OrMoreDomestic
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Interest expense incurred on foreign time deposits, including certificate of deposits, of 100,000 dollars or more in domestic offices.

No definition available.

Name:	iboc_InterestExpenseTimeDeposits100000OrMoreForeign
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

No definition available.

Name:	iboc_InterestExpenseTimeDepositsLessThan100000Abstract
Namespace Prefix:	iboc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Interest expense incurred on domestic time deposits, including certificate of deposits, less than 100,000 dollars in domestic offices.

No definition available.

Name:	iboc_InterestExpenseTimeDepositsLessThan100000Domestic
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Interest expense incurred on foreign time deposits, including certificate of deposits, less than 100,000 dollars in domestic offices.

No definition available.

Name:	iboc_InterestExpenseTimeDepositsLessThan100000Foreign
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

No definition available.

Name:	iboc_TimeDepositsLessThan100000Abstract
Namespace Prefix:	iboc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The aggregate amount of domestic time deposits (including certificates of deposits, individual retirement accounts and open accounts) that total to less than 100,000 dollars.

No definition available.

Name:	iboc_TimeDepositsLessThan100000Domestic
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The aggregate amount of foreign time deposits (including certificates of deposits, individual retirement accounts and open accounts) that total to less than 100,000 dollars.

No definition available.

Name:	iboc_TimeDepositsLessThan100000Foreign
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

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

No definition available.

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

The aggregate of all domestic and foreign interest-bearing deposit liabilities.

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

No definition available.

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

Total amount of interest-bearing domestic deposit liabilities, which may include brokered, retail, demand, checking, notice of withdrawal, money market and other interest-bearing deposits.

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

Amount of foreign interest-bearing deposits held by the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05(a))
-URI https://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

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 210.9-03.12)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

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

Aggregate amount of interest expense on all deposits.

Name:	us-gaap_InterestExpenseDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The aggregate interest expense incurred on the following for deposits in domestic offices: (1) transaction accounts (NOW, and telephone and preauthorized transfer accounts), (2) money market deposits and (3) savings deposits.

Name:	us-gaap_InterestExpenseNOWAccountsMoneyMarketAccountsAndSavingsDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

No definition available.

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

The aggregate interest expense incurred on time deposits, including certificates of deposits, in domestic offices.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.6)
-URI https://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph (a)

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph (b)
-Subparagraph (1)

Name:	us-gaap_InterestExpenseTimeDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

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

No definition available.

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

Amount of domestic noninterest-bearing deposits held by the entity, which may include demand deposits, checking, brokered and retail deposits.

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

Amount of foreign noninterest-bearing deposits held by the entity which may include demand deposits, checking, brokered and retail deposits.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05)
-URI https://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

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 210.9-03.12)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

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

Amount of time deposit liabilities, including certificates of deposit.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

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 210.9-03.12)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

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

Amount of time deposit liabilities, including certificates of deposit, in denominations of $100,000 or more.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Industry Guide
-Number 3
-Section V
-Paragraph D-E