Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	$ 3,068	$ 4,365
Loans, net of allowance for loan losses, Carrying Amount	4,595,494	4,351,539
Securities held for investment, Fair Value	3,098	4,403
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,302,046	6,248,057
Short-term borrowings, Carrying Amount	2,100	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	1,855	1,718
Letters of credit, Fair Value	446	424
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,728,943	1,851,161
Securities held for investment, Carrying Amount	2,568	3,722
Cash and cash equivalents, Fair Value	1,728,943	1,851,161
Securities held for investment, Fair Value	2,598	3,760
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,302,046	6,248,057
Short-term borrowings, Carrying Amount	2,100	500
Junior subordinated debentures, Carrying Amount	31,959	31,959
Deposits, Fair Value	6,374,232	6,313,622
Short-term borrowings, Fair Value	2,100	500
Junior subordinated debentures, Fair Value	33,589	34,339
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	500	643
Loans, net of allowance for loan losses, Carrying Amount	4,595,494	4,351,539
Securities held for investment, Fair Value	500	643
Loans, net of allowance for loan losses, Fair Value	$ 4,599,161	$ 4,367,363

Cash and cash equivalents and federal funds sold fair value disclosure.

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Financial instruments off balance sheet.

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Letters of credit fair value disclosure.

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The sum of cash and cash equivalents plus Federal Funds Sold. Cash and cash equivalents consist of short term, highly liquid investments that are readily convertible to known amounts of cash and are so near their maturity that they present negligible risk of changes in value due to changes in interest rates -- usually with an original maturity less than 90 days.

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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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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

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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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Fair value 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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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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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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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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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

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