Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
FINANCIAL ASSETS
Securities held for investment	$ 8,333	$ 8,593
Loans, net of allowance for loan losses	3,918,699	3,810,509
FINANCIAL LIABILITIES
Deposits	5,698,591	5,904,704
Short-term borrowings	3,777	3,982
Junior subordinated debentures	26,804	26,804
Carrying Amount [Member] \| Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents	1,655,061	1,913,895
Securities held for investment	8,333	8,593
FINANCIAL LIABILITIES
Deposits	5,698,591	5,904,704
Short-term borrowings	3,777	3,982
Junior subordinated debentures	26,804	26,804
Carrying Amount [Member] \| Level 3 Inputs [Member]
FINANCIAL ASSETS
Loans, net of allowance for loan losses	3,918,699	3,810,509
Fair Value [Member]
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments	1,708	1,640
Letters of credit	475	478
Fair Value [Member] \| Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents	1,655,061	1,913,895
Securities held for investment	8,398	8,671
FINANCIAL LIABILITIES
Deposits	5,750,163	5,945,502
Short-term borrowings	3,777	3,982
Junior subordinated debentures	28,123	31,200
Fair Value [Member] \| Level 3 Inputs [Member]
FINANCIAL ASSETS
Loans, net of allowance for loan losses	$ 3,965,439	$ 3,847,791

Financial instruments off balance sheet.

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Fair value portion 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.

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Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

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

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Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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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. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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