Investment Securities (Schedule Of Securities With Gross Unrealized Losses Aggregated By Investment Category) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	$ (988)	$ (560)
Securities available for sale, Less Than Twelve Months, Fair Value	234,030	158,903
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(349)	(360)
Securities available for sale, Over Twelve Months, Fair Value	17,943	22,764
Securities available for sale, Gross Unrealized Losses, Total	(1,337)	(920)
Securities available for sale, Fair Value, Total	251,973	181,667
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses	(73)	(26)
Securities held to maturity, Less Than Twelve Months, Fair Value	10,452	2,354
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses		(31)
Securities held to maturity, Over Twelve Months, Fair Value		1,297
Securities held to maturity, Gross Unrealized Losses, Total	(73)	(57)
Securities held to maturity, Fair Value, Total	10,452	3,651
U.S. Government-Sponsored Enterprise Obligations [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(167)	(4)
Securities available for sale, Less Than Twelve Months, Fair Value	67,848	9,996
Securities available for sale, Gross Unrealized Losses, Total	(167)	(4)
Securities available for sale, Fair Value, Total	67,848	9,996
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses
Securities held to maturity, Less Than Twelve Months, Fair Value
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total
Securities held to maturity, Fair Value, Total
Obligations Of State And Political Subdivisions [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(6)	(11)
Securities available for sale, Less Than Twelve Months, Fair Value	334	1,104
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(185)	(187)
Securities available for sale, Over Twelve Months, Fair Value	1,087	1,085
Securities available for sale, Gross Unrealized Losses, Total	(191)	(198)
Securities available for sale, Fair Value, Total	1,421	2,189
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses	(73)	(26)
Securities held to maturity, Less Than Twelve Months, Fair Value	10,452	2,354
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses		(31)
Securities held to maturity, Over Twelve Months, Fair Value		1,297
Securities held to maturity, Gross Unrealized Losses, Total	(73)	(57)
Securities held to maturity, Fair Value, Total	10,452	3,651
Mortgage Backed Securities [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(815)	(545)
Securities available for sale, Less Than Twelve Months, Fair Value	165,848	147,803
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(164)	(173)
Securities available for sale, Over Twelve Months, Fair Value	16,856	21,679
Securities available for sale, Gross Unrealized Losses, Total	(979)	(718)
Securities available for sale, Fair Value, Total	182,704	169,482
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses
Securities held to maturity, Less Than Twelve Months, Fair Value
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total
Securities held to maturity, Fair Value, Total

Name:	ibkc_ScheduleOfSecuritiesAvailableForSaleAndHeldToMaturityLineItems
Namespace Prefix:	ibkc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for those securities which are categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the excess of amortized cost basis over fair value of securities in a loss position and which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563

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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 03-1

-Paragraph 21

-Subparagraph a

-Clause 2

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 17

-Subparagraph a

-Clause 2

-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_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 03-1

-Paragraph 21

-Subparagraph a(2)

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 17

-Subparagraph a(1)

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for those securities which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 17

-Subparagraph a

-Clause 2

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27337-111563

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563

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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 03-1

-Paragraph 21

-Subparagraph a

-Clause 2

-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_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27337-111563

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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 03-1

-Paragraph 21

-Subparagraph a(2)

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 17

-Subparagraph a(1)

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the excess of amortized cost basis over fair value of securities in a loss position and which are categorized as held-to-maturity.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant