Loss Sharing Agreements and FDIC Loss Share Receivable - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2013	Dec. 31, 2014 Business	Dec. 31, 2013
Receivables [Abstract]
Percentage of covered loan and foreclosed real estate losses		80.00%
Losses that exceed contractual thresholds		95.00%
Number of acquisitions		4
Recovery period for covered assets		3 years
Liability for reserve for amount of consideration			$ 800,000
FDIC loss share receivables		69,627,000us-gaap_FDICIndemnificationAsset	162,312,000us-gaap_FDICIndemnificationAsset
Valuation allowance against indemnification assets	$ 31,800,000	$ 5,121,000	$ 31,813,000

Federal deposit insurance corporation indemnification asset claw back liability net.

No definition available.

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- Definition

Loan recovery period.

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No definition available.

+ Details

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- Definition

Total number of business acquired by the entity with indemnification agreements that were acquired in previous periods

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No definition available.

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Period Type:	duration

- Definition

Percentage acquired by FDIC in first amount exceeding losses covered assets.

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No definition available.

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Namespace Prefix:	ibkc_
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- Definition

Percentage of loan losses covered under loss share agreement.

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No definition available.

+ Details

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Data Type:	num:percentItemType
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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.

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No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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).

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No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Details

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