Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	$ 1,997	$ 1,428
Loans, net of allowance for loan losses, Carrying Amount	5,039,295	4,924,587
Securities held for investment, Fair Value	2,003	1,433
Loans held for sale, Fair Value	10,899	8,174
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,613,613	6,605,495
Short-term borrowings, Carrying Amount	1,000	1,675
Junior subordinated debentures, Carrying Amount	26,804	26,804
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	2,277	2,158
Letters of credit, Fair Value	414	421
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,704,438	1,424,255
Securities held for investment, Carrying Amount	1,497	928
Loans held for sale, Carrying Amount	10,899	8,174
Cash and cash equivalents, Fair Value	1,704,438	1,424,255
Securities held for investment, Fair Value	1,503	933
Loans held for sale, Fair Value	10,899	8,174
FINANCIAL LIABILITIES
Deposits, Carrying Amount	6,613,613	6,605,495
Short-term borrowings, Carrying Amount	1,000	1,675
Junior subordinated debentures, Carrying Amount	26,804	26,804
Deposits, Fair Value	6,729,518	6,713,542
Short-term borrowings, Fair Value	1,000	1,675
Junior subordinated debentures, Fair Value	28,777	29,549
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	500	500
Loans, net of allowance for loan losses, Carrying Amount	5,039,295	4,924,587
Securities held for investment, Fair Value	500	500
Loans, net of allowance for loan losses, Fair Value	$ 5,052,074	$ 4,901,159

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 of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (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 loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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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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The face amount of loans that will be sold to other entities.

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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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