Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Sep. 30, 2023 USD ($) Security	Dec. 31, 2022 USD ($) Security
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	642	2,646
Less than 12 months, Fair Value	$ 407,375	$ 3,654,393
Less than 12 months, Unrealized Losses	$ (20,634)	$ (198,376)
12 months or more, Count | Security	2,766	674
12 months or more, Fair Value	$ 5,802,575	$ 3,098,117
12 months or more, Unrealized Losses	$ (897,779)	$ (575,017)
Total Count | Security	3,408	3,320
Total Fair Value	$ 6,209,950	$ 6,752,510
Total Unrealized Losses	$ (918,413)	$ (773,393)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	6	61
Less than 12 months, Fair Value	$ 42,600	$ 688,208
Less than 12 months, Unrealized Losses	$ (322)	$ (22,731)
12 months or more, Count | Security	63	4
12 months or more, Fair Value	$ 736,775	$ 55,314
12 months or more, Unrealized Losses	$ (17,541)	$ (4,415)
Total Count | Security	69	65
Total Fair Value	$ 779,375	$ 743,522
Total Unrealized Losses	$ (17,863)	$ (27,146)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security		27
Less than 12 months, Fair Value		$ 140,877
Less than 12 months, Unrealized Losses		$ (4,734)
12 months or more, Count | Security	29	2
12 months or more, Fair Value	$ 173,350	$ 30,419
12 months or more, Unrealized Losses	$ (4,687)	$ (2,231)
Total Count | Security	29	29
Total Fair Value	$ 173,350	$ 171,296
Total Unrealized Losses	$ (4,687)	$ (6,965)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	18	687
Less than 12 months, Fair Value	$ 27,783	$ 1,415,169
Less than 12 months, Unrealized Losses	$ (1,595)	$ (102,881)
12 months or more, Count | Security	856	205
12 months or more, Fair Value	$ 3,452,081	$ 2,557,035
12 months or more, Unrealized Losses	$ (693,694)	$ (489,994)
Total Count | Security	874	892
Total Fair Value	$ 3,479,864	$ 3,972,204
Total Unrealized Losses	$ (695,289)	$ (592,875)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	617	1,744
Less than 12 months, Fair Value	$ 332,747	$ 936,865
Less than 12 months, Unrealized Losses	$ (18,712)	$ (51,427)
12 months or more, Count | Security	1,517	273
12 months or more, Fair Value	$ 864,436	$ 233,679
12 months or more, Unrealized Losses	$ (141,780)	$ (53,372)
Total Count | Security	2,134	2,017
Total Fair Value	$ 1,197,183	$ 1,170,544
Total Unrealized Losses	$ (160,492)	$ (104,799)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security		86
Less than 12 months, Fair Value		$ 146,615
Less than 12 months, Unrealized Losses		$ (8,783)
12 months or more, Count | Security	267	189
12 months or more, Fair Value	$ 344,901	$ 216,885
12 months or more, Unrealized Losses	$ (38,013)	$ (24,776)
Total Count | Security	267	275
Total Fair Value	$ 344,901	$ 363,500
Total Unrealized Losses	$ (38,013)	$ (33,559)
Collateralized loan obligations [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	1	41
Less than 12 months, Fair Value	$ 4,245	$ 326,659
Less than 12 months, Unrealized Losses	$ (5)	$ (7,820)
12 months or more, Count | Security	34	1
12 months or more, Fair Value	$ 231,032	$ 4,785
12 months or more, Unrealized Losses	$ (2,064)	$ (229)
Total Count | Security	35	42
Total Fair Value	$ 235,277	$ 331,444
Total Unrealized Losses	$ (2,069)	$ (8,049)