Derivative Instruments and Hedging Activities - Outstanding Interest Rate Swap Agreements Not Designated as Hedging Instruments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Offsetting [Abstract]
Weighted average pay rate	2.90%ibkc_WeightedAveragePayRate	3.00%ibkc_WeightedAveragePayRate
Weighted average receive rate	0.40%ibkc_WeightedAverageReceiveRate	0.20%ibkc_WeightedAverageReceiveRate
Weighted average maturity in years	7 years 8 months 12 days	7 years 7 months 6 days
Unrealized gain (loss) relating to interest rate swaps	$ 0us-gaap_UnrealizedGainLossOnDerivatives	$ 0us-gaap_UnrealizedGainLossOnDerivatives

The weighted average rate paid on derivative instruments in the current period.

No definition available.

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- Definition

The weighted average rate paid to the Company on derivative instruments in the current period.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ 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

+ Details

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