Securities Sold Under Repurchase Agreements (Details) (USD $)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Securities Sold Under Repurchase Agreements
Average outstanding amount	$ 1,415,769,000	$ 1,479,734,000
Maximum amount outstanding at any month end	1,437,480,000	1,520,714,000
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	1,672,925,000	1,747,043,000
Collateralized securities, Fair Value of Securities Sold	1,736,519,000	1,777,975,000
Repurchase Borrowing, Balance of Liability	1,348,629,000	1,433,270,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.02%	2.97%
Overnight agreements
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	377,620,000	432,252,000
Collateralized securities, Fair Value of Securities Sold	388,304,000	426,112,000
Repurchase Borrowing, Balance of Liability	245,026,000	311,333,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.32%	0.60%
1 to 29 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	13,772,000	26,714,000
Collateralized securities, Fair Value of Securities Sold	14,227,000	27,177,000
Repurchase Borrowing, Balance of Liability	10,263,000	16,308,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	1.13%	1.15%
30 to 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	45,512,000	44,443,000
Collateralized securities, Fair Value of Securities Sold	48,177,000	45,438,000
Repurchase Borrowing, Balance of Liability	30,095,000	30,167,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	1.12%	1.69%
Over 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	1,236,021,000	1,243,634,000
Collateralized securities, Fair Value of Securities Sold	1,285,811,000	1,279,248,000
Repurchase Borrowing, Balance of Liability	$ 1,063,245,000	$ 1,075,462,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.72%	3.71%

Represents the average amount of securities sold under repurchase agreements during the period.

No definition available.

Name:	iboc_SecuritiesSoldUnderAgreementsToRepurchaseAverageOutstandingAmount
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the maximum balance outstanding at any month end during the period as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Aggregate carrying value of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets, as of the most recent balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph m

-Subparagraph 1

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

+ Details

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

- Definition

The interest rate associated with the repurchase agreements. This is also known as the repo rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph m

-Subparagraph 1

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

+ Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

This is the value on the open market of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets, as of the most recent balance sheet date.

+ References

+ Details

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

- Definition

The carrying value as of the balance sheet date of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(1))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=6935251&loc=d3e9760-107771

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Subparagraph 1

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(a)(i))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 100

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Chapter 4

-Paragraph 36

-IssueDate 2006-05-01

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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