Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 206,454	$ 16,963	$ 391,054
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,722	42	13,114
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,276,936	4,655,649	4,576,738
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	558,196	511,822	608,383
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	4,483,390	4,672,612	4,967,792
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	559,918	511,864	621,497
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	1,448	3,477	47,498
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	12	7	1,614
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	340,528	477,306	436,278
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	10,694	14,738	22,734
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	341,976	480,783	483,776
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	10,706	14,745	24,348
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	32,423	11,855	137,728
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	320	34	1,838
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,391,496	1,427,975	1,457,431
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	139,117	125,148	164,796
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	1,423,919	1,439,830	1,595,159
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	139,437	125,182	166,634
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	80,096	1,631	68,439
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	462	1	3,609
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,183,346	2,361,089	2,403,837
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	389,031	352,882	399,151
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	2,263,442	2,362,720	2,472,276
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	389,493	352,883	402,760
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	92,487	0	99,347
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	928	0	4,771
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	257,535	284,324	215,040
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11,054	11,339	12,671
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	350,022	284,324	314,387
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	11,982	11,339	17,442
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	0	0	38,042
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	0	1,282
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	104,031	104,955	64,152
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	8,300	7,715	9,031
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	104,031	104,955	102,194
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 8,300	$ 7,715	$ 10,313