Securities - Gross Unrealized Losses and Fair Value of Investment Securities Held to Maturity (Detail) $ in Thousands	Mar. 31, 2024 USD ($) Security	Dec. 31, 2023 USD ($) Security
Schedule Of Held To Maturity Securities [Line Items]
Less than 12 months, Count | Security	76	148
Less than 12 months, Fair Value	$ 255,618	$ 572,419
Less than 12 months,Unrealized Losses	$ (11,420)	$ (22,571)
12 months or more, Count | Security	1,700	1,617
12 months or more, Fair Value	$ 4,303,905	$ 4,110,888
12 months or more, Unrealized Losses	$ (605,836)	$ (506,969)
Total Count | Security	1,776	1,765
Total Fair Value	$ 4,559,523	$ 4,683,307
Total Unrealized Losses	$ (617,256)	$ (529,540)
U.S. Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
12 months or more, Count | Security	11	11
12 months or more, Fair Value	$ 120,579	$ 120,524
12 months or more, Unrealized Losses	$ (2,661)	$ (2,686)
Total Count | Security	11	11
Total Fair Value	$ 120,579	$ 120,524
Total Unrealized Losses	$ (2,661)	$ (2,686)
Mortgage-backed [Member]
Schedule Of Held To Maturity Securities [Line Items]
Less than 12 months, Count | Security	6	2
Less than 12 months, Fair Value	$ 3,618	$ 1,469
Less than 12 months,Unrealized Losses	$ (56)	$ (14)
12 months or more, Count | Security	263	263
12 months or more, Fair Value	$ 2,282,995	$ 2,377,922
12 months or more, Unrealized Losses	$ (404,464)	$ (356,643)
Total Count | Security	269	265
Total Fair Value	$ 2,286,613	$ 2,379,391
Total Unrealized Losses	$ (404,520)	$ (356,657)
State and political subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Less than 12 months, Count | Security	70	146
Less than 12 months, Fair Value	$ 252,000	$ 570,950
Less than 12 months,Unrealized Losses	$ (11,364)	$ (22,557)
12 months or more, Count | Security	1,426	1,343
12 months or more, Fair Value	$ 1,900,331	$ 1,612,442
12 months or more, Unrealized Losses	$ (198,711)	$ (147,640)
Total Count | Security	1,496	1,489
Total Fair Value	$ 2,152,331	$ 2,183,392
Total Unrealized Losses	$ (210,075)	$ (170,197)