Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Securities held-to-maturity		$ 2,509,088	$ 2,351,120
Loans		2,960,343	2,396,534
Financial liabilities:
Time deposits		546,143	555,447
Other borrowed funds		177,009	370,955
Subordinated debentures		36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity		2,509,088	2,351,120
Loans	[1]	2,960,343	2,396,534
Financial liabilities:
Time deposits		546,143	555,447
Other borrowed funds		177,009	370,955
Subordinated debentures		36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity		2,579,103	2,361,304
Loans	[1]	2,902,390	2,424,770
Financial liabilities:
Time deposits		556,470	560,746
Other borrowed funds		183,000	374,531
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity		2,579,103	2,361,304
Financial liabilities:
Time deposits		556,470	560,746
Other borrowed funds		183,000	374,531
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	[1]	$ 2,902,390	$ 2,424,770

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