Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial Assets
Cash and cash equivalents	$ 548,095	$ 391,396	$ 970,977	$ 573,296
Loans and loans held for sale, net of unearned income	11,581,116ibkc_LoansAndLoansHeldForSale	9,620,461ibkc_LoansAndLoansHeldForSale	8,766,055ibkc_LoansAndLoansHeldForSale
FDIC loss share receivable	69,627us-gaap_FDICIndemnificationAsset	162,312us-gaap_FDICIndemnificationAsset
Financial Liabilities
Deposits	12,520,525us-gaap_Deposits	10,737,000us-gaap_Deposits	10,748,277us-gaap_Deposits
Short-term borrowings	845,742us-gaap_ShortTermBorrowings	680,344us-gaap_ShortTermBorrowings	303,045us-gaap_ShortTermBorrowings
Long-term debt	403,254us-gaap_LongTermDebt	280,699us-gaap_LongTermDebt
Carrying Amount [Member]
Financial Assets
Cash and cash equivalents	548,095	391,396
Investment securities	2,275,813	2,090,906
Loans and loans held for sale, net of unearned income	11,581,116	9,620,461
FDIC loss share receivable	69,627	162,312
Derivative instruments	32,903	30,076
Accrued interest receivable	37,696	32,143
Financial Liabilities
Deposits	12,520,525	10,737,000
Short-term borrowings	845,742	680,344
Long-term debt	403,254	280,699
Derivative instruments	31,354	26,735
Accrued interest payable	8,258	6,102
Fair Value [Member]
Financial Assets
Cash and cash equivalents	548,095	391,396
Investment securities	2,278,334	2,089,363
Loans and loans held for sale, net of unearned income	11,605,446	9,724,432
FDIC loss share receivable	19,606	21,918
Derivative instruments	32,903	30,076
Accrued interest receivable	37,696	32,143
Financial Liabilities
Deposits	12,298,017	10,226,573
Short-term borrowings	845,742	680,344
Long-term debt	376,139	235,503
Derivative instruments	31,354	26,735
Accrued interest payable	$ 8,258	$ 6,102

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Loans and loans held for sale.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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