Fair Value Measurements - Estimated Fair Values of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
FINANCIAL ASSETS
Federal funds sold, Carrying Amount		$ 1,000
Securities held for investment, Carrying Amount	$ 2,971	1,903
Loans, net of allowance for credit losses,Carrying Amount	6,505,967	5,607,905
Securities held for investment, Fair Value	3,011	1,903
Loans held for sale, Fair Value	48,601	11,001
FINANCIAL LIABILITIES
Deposits, Carrying Amount	8,495,891	7,483,635
Short-term borrowings, Carrying Amount	300	1,100
Junior subordinated debentures, Carrying Amount	26,804	26,804
OFF-BALANCE SHEET FINANCIAL INSTRUMENTS
Loan commitments, Fair Value	3,091	2,832
Letters of credit, Fair Value	518	485
Level 2 Inputs [Member]
FINANCIAL ASSETS
Cash and cash equivalents, Carrying Amount	1,835,524	1,868,281
Federal funds sold, Carrying Amount		1,000
Securities held for investment, Carrying Amount	66	1,403
Loans held for sale, Carrying Amount	48,601	11,001
Cash and cash equivalents, Fair Value	1,835,524	1,868,281
Federal funds sold, Fair Value		1,000
Securities held for investment, Fair Value	69	1,403
Loans held for sale, Fair Value	48,601	11,001
FINANCIAL LIABILITIES
Deposits, Carrying Amount	8,495,891	7,483,635
Short-term borrowings, Carrying Amount	300	1,100
Junior subordinated debentures, Carrying Amount	26,804	26,804
Deposits, Fair Value	8,512,546	7,497,429
Short-term borrowings, Fair Value	300	1,100
Junior subordinated debentures, Fair Value	30,771	29,324
Level 3 Inputs [Member]
FINANCIAL ASSETS
Securities held for investment, Carrying Amount	2,905	500
Loans, net of allowance for credit losses,Carrying Amount	6,505,967	5,607,905
Securities held for investment, Fair Value	2,942	500
Loans, net of allowance for credit losses,Fair Value	$ 6,560,188	$ 5,625,005

Financial instruments off balance sheet.

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Letters of credit fair value disclosure.

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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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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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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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The amount outstanding of funds lent to other depository institutions, securities brokers, or securities dealers in the form of Federal Funds sold; for example, immediately available funds lent under agreements or contracts that mature in one business day or roll over under a continuing contract, regardless of the nature of the transaction or the collateral involved, excluding overnight lending for commercial and industrial purposes. Also include Federal Funds sold under agreements to resell on a gross basis, excluding (1) sales of term Federal Funds, (2) due bills representing purchases of securities or other assets by the reporting bank that have not yet been delivered and similar instruments, (3) resale agreements that mature in more than one business day involving assets other than securities, and (4) yield maintenance dollar repurchase agreements.

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Fair value portion of federal funds sold and securities borrowed or purchased under agreements to resell.

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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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Amount, before valuation allowance, of financing receivable classified as held-for-sale.

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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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