Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Securities held-to-maturity		$ 3,211,977	$ 2,509,088
Loans		2,890,713	2,960,343
Financial liabilities:
Time deposits		348,296	546,143
Other borrowed funds		118,786	177,009
Subordinated debentures		36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity		3,211,977	2,509,088
Loans	[1]	2,890,713	2,960,343
Financial liabilities:
Time deposits		348,296	546,143
Other borrowed funds		118,786	177,009
Subordinated debentures		36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity		3,207,259	2,579,103
Loans	[1]	2,796,258	2,902,390
Financial liabilities:
Time deposits		345,677	556,470
Other borrowed funds		122,013	183,000
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity		3,207,259	2,579,103
Financial liabilities:
Time deposits		345,677	556,470
Other borrowed funds		122,013	183,000
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	[1]	$ 2,796,258	$ 2,902,390

[1] Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.