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Allowance for Probable Loan Losses (Details 4) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Loan loss allowances, impaired financing receivable
Period of charge off for past due unsecured commercial loans (in days)	90
Commercial | Loans with Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	$ 23,018	$ 23,062
Unpaid Principal Balance	23,061	23,071
Related Allowance	8,049	8,138
Average Recorded Investment	23,084	23,096
Interest Recognized	30	42
Commercial | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	655	364
Unpaid Principal Balance	665	980
Average Recorded Investment	662	993
Interest Recognized		30
Commercial real estate: other construction and land development | Loans with Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	35,250	10,603
Unpaid Principal Balance	35,272	10,645
Related Allowance	3,000	592
Average Recorded Investment	35,248	10,622
Commercial real estate: other construction and land development | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	26,036	66,604
Unpaid Principal Balance	26,036	66,755
Average Recorded Investment	27,680	68,608
Interest Recognized	58	2
Commercial real estate: farmland and commercial | Loans with Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	29,915	17,841
Unpaid Principal Balance	29,929	17,878
Related Allowance	7,923	3,441
Average Recorded Investment	24,160	18,475
Interest Recognized	607	860
Commercial real estate: farmland and commercial | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	17,386	4,003
Unpaid Principal Balance	17,832	5,606
Average Recorded Investment	17,145	5,594
Interest Recognized	48
Commercial real estate: multifamily | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	431	473
Unpaid Principal Balance	431	473
Average Recorded Investment	447	500
Residential: first lien | Loans with Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	389
Unpaid Principal Balance	389
Related Allowance	94
Average Recorded Investment	389
Residential: first lien | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	1,792	2,015
Unpaid Principal Balance	1,847	2,143
Average Recorded Investment	1,629	2,297
Interest Recognized	14
Residential: junior lien | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	1,722	199
Unpaid Principal Balance	1,729	226
Average Recorded Investment	1,708	228
Interest Recognized	86
Consumer | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	1,299	29
Unpaid Principal Balance	1,301	46
Average Recorded Investment	1,624	49
Foreign | Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment		7
Unpaid Principal Balance		7
Average Recorded Investment		19
Loans with Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	88,572	51,506
Unpaid Principal Balance	88,651	51,594
Related Allowance	19,066	12,171
Average Recorded Investment	82,881	52,193
Interest Recognized	637	902
Loans with No Related Allowance
Loan loss allowances, impaired financing receivable
Recorded Investment	49,321	73,694
Unpaid Principal Balance	49,841	76,236
Average Recorded Investment	50,895	78,288
Interest Recognized	$ 206	$ 32

X

- Definition

Represents the period of charge off for past due unsecured commercial loans.

+ Details

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

X

- 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.

+ Details

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The average recorded investment related to impaired financing receivables.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524

-Subparagraph (c)(1)



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5258-111524

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The interest income recognized during the time within that period that the financing receivables were impaired.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524

-Subparagraph (c)(2)

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Namespace Prefix:	us-gaap
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Balance Type:	credit
Period Type:	duration

X

- Definition

The recorded investment related to impaired financing receivables.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-URI http://asc.fasb.org/extlink&oid=6965416&loc=SL6953550-111524

-Subparagraph (b)

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524

-Subparagraph (a)(3)

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The unpaid principal balance related to impaired financing receivables.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524

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