Transfers and Servicing of Financial Assets (Including Mortgage Banking Activity) - Schedule of Related Amortization Expense of Mortgage Servicing Rights (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Identifiable Intangible Assets Acquired As Part Of Business Combination [Line Items]
Aggregate amortization expense	$ 5,807us-gaap_AmortizationOfIntangibleAssets	$ 4,720us-gaap_AmortizationOfIntangibleAssets	$ 5,150us-gaap_AmortizationOfIntangibleAssets
2015	5,646
2016	5,187
2017	3,586
2018	2,597
2019	1,877
2020 and thereafter	1,468
Mortgage Servicing Rights [Member]
Schedule Of Identifiable Intangible Assets Acquired As Part Of Business Combination [Line Items]
Aggregate amortization expense	759	480	225
2015	1,550
2016	697
2017	531
2018	371
2019	230
2020 and thereafter	$ 119

Transfers and Servicing of Financial Assets (Including Mortgage Banking Activity) - Schedule of Related Amortization Expense of Mortgage Servicing Rights (Detail) (USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Schedule Of Identifiable Intangible Assets Acquired As Part Of Business Combination [Line Items]

Aggregate amortization expense

$ 5,807us-gaap_AmortizationOfIntangibleAssets

$ 4,720us-gaap_AmortizationOfIntangibleAssets

$ 5,150us-gaap_AmortizationOfIntangibleAssets

2015

5,646us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths

2016

5,187us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo

2017

3,586us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree

2018

2,597us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour

2019

1,877us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive

2020 and thereafter

1,468us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive

Mortgage Servicing Rights [Member]

Schedule Of Identifiable Intangible Assets Acquired As Part Of Business Combination [Line Items]

Aggregate amortization expense

759us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

480us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

225us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2015

1,550us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2016

697us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2017

531us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2018

371us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2019

230us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

2020 and thereafter

$ 119us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= ibkc_MortgageServicingRightsMember

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

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- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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