Derivative Instruments and Other Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Derivatives, Fair Value [Line Items]
Cash as collateral for derivative transactions	$ 16.1	$ 11.5
Fair value of derivatives	0.4ibkc_FairValueOfDerivatives
Cash Flow Hedging [Member]
Derivatives, Fair Value [Line Items]
Amount of Gain (Loss) recognized in other comprehensive income or reclassified from Accumulated OCI	$ 0		$ 0

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4D

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624177-113959

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