Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Securities held-to-maturity	$ 2,110,555	$ 2,046,647
Loans	2,281,812	2,257,035
Financial liabilities:
Time deposits	542,079	560,579
Other borrowed funds	257,148	202,378
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	2,110,555	2,046,647
Loans	2,310,660	2,257,035
Financial liabilities:
Time deposits	542,079	560,579
Other borrowed funds	257,148	202,378
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	2,074,443	1,991,421
Loans	2,321,747	2,279,712
Financial liabilities:
Time deposits	542,733	559,988
Other borrowed funds	259,045	203,122
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	2,074,443	1,991,421
Financial liabilities:
Time deposits	542,733	559,988
Other borrowed funds	259,045	203,122
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	$ 2,321,747	$ 2,279,712

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Amount of investment in debt security measured at amortized cost (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 time deposit liabilities, including certificates of deposit.

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