Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financial assets:
Securities held-to-maturity	$ 1,406,792us-gaap_HeldToMaturitySecurities	$ 1,487,884us-gaap_HeldToMaturitySecurities
Loans	1,309,048	1,243,822
Financial liabilities:
Time deposits	383,145us-gaap_TimeDeposits	382,224us-gaap_TimeDeposits
Other borrowed funds	395,500us-gaap_OtherBorrowings	255,144us-gaap_OtherBorrowings
Subordinated debentures	36,083us-gaap_SubordinatedDebt	36,083us-gaap_SubordinatedDebt
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	1,406,792	1,487,884
Loans	1,309,048	1,243,822
Financial liabilities:
Time deposits	383,145	382,224
Other borrowed funds	395,500	255,144
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	1,413,603	1,464,449
Loans	1,291,550	1,214,192
Financial liabilities:
Time deposits	387,919	386,742
Other borrowed funds	400,196	254,736
Subordinated debentures	36,083	39,503
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	1,413,603	1,464,449
Financial liabilities:
Time deposits	387,919	386,742
Other borrowed funds	400,196	254,736
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	1,291,550	1,214,192
Financial liabilities:
Subordinated debentures	$ 36,083	$ 39,503

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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Amount of deposits that cannot be withdrawn before a set date without penalty or for which notice of withdrawal is required. Time deposits include, but are not limited to, certificates of deposits, individual retirement accounts and open accounts.

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