Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 249,374	$ 456,673	$ 206,454
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(3,745)	(8,507)	(1,722)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	3,740,924	3,967,472	4,276,936
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(480,523)	(529,712)	(558,196)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	3,990,298	4,424,145	4,483,390
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(484,268)	(538,219)	(559,918)
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	1,497	0	1,448
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	0	(12)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	151,350	271,088	340,528
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(1,876)	(4,595)	(10,694)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	152,847	271,088	341,976
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(1,876)	(4,595)	(10,706)
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	74,621	56,777	32,423
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(2,300)	(1,224)	(320)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,308,960	1,361,628	1,391,496
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(171,232)	(137,618)	(139,117)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	1,383,581	1,418,405	1,423,919
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(173,532)	(138,842)	(139,437)
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	166,062	198,385	80,096
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(1,315)	(4,990)	(462)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,011,749	2,048,727	2,183,346
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(298,833)	(373,756)	(389,031)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	2,177,811	2,247,112	2,263,442
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(300,148)	(378,746)	(389,493)
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	7,194	196,640	92,487
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(130)	(2,249)	(928)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	181,352	190,189	257,535
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(4,306)	(7,409)	(11,054)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	188,546	386,829	350,022
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(4,436)	(9,658)	(11,982)
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	4,871	0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	(44)	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	87,513	95,840	104,031
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(4,276)	(6,334)	(8,300)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	87,513	100,711	104,031
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ (4,276)	$ (6,378)	$ (8,300)