Loss Sharing Agreements and FDIC Loss Share Receivable - Schedule of FDIC Loss Share Receivables (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
FDIC Loss Share Receivable [Line Items]
Balance at beginning of period		$ 162,312us-gaap_FDICIndemnificationAsset
Change due to (reversal of) loan loss provision recorded on FDIC covered loans		(4,260)	(56,085)	84,085
Amortization		(74,617)	(97,849)	(118,100)
Impairment	31,800	5,121	31,813
Balance at end of period		69,627us-gaap_FDICIndemnificationAsset	162,312us-gaap_FDICIndemnificationAsset
FDIC Loss Share Receivables [Member]
FDIC Loss Share Receivable [Line Items]
Balance at beginning of period	423,069	162,312	423,069
Change due to (reversal of) loan loss provision recorded on FDIC covered loans		(4,260)	(56,085)
Amortization		(74,617)	(97,849)
Recoveries payable (submission of reimbursable losses) to the FDIC		3,282	(52,586)
Impairment		(5,121)	(31,813)
Changes due to a change in cash flow assumptions on OREO and other changes		(11,969)	(22,424)
Balance at end of period		$ 69,627	$ 162,312

- Definition

Increase or decrease in the indemnification asset due to the change in expected losses during the current period of loans covered by loss share agreements.

+ References

No definition available.

+ Details

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Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Change in cash flow assumptions.

+ References

No definition available.

+ Details

Name:	ibkc_ChangeInCashFlowAssumptions
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

Decrease in the fair value based on submitted claims to the FDIC of actual losses incurred during the period on assets covered by loss share agreements.

+ References

No definition available.

+ Details

Name:	ibkc_SubmissionOfReimbursableLossesToFdic
Namespace Prefix:	ibkc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying amount of receivable assets originating from the indemnification agreement with the Federal Deposit Insurance Corporation (FDIC), in which the FDIC is to partially cover losses.

+ References

No definition available.

+ Details

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

- Definition

Increases to the indemnification asset due to accretion of the loan discount relating to loss sharing agreements with the Federal Deposit Insurance Corporation (FDIC).

+ References

No definition available.

+ Details

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

- Definition

Amount of decreases to the indemnification asset due to write offs of loans, net of recoveries, relating to loss sharing agreements with the Federal Deposit Insurance Corporation (FDIC).

+ References

No definition available.

+ Details

Name:	us-gaap_FDICIndemnificationAssetNetWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: