Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 456,673	$ 16,963
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(8,507)	(42)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	3,967,472	4,655,649
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(529,712)	(511,822)
Available-for-sale securities, continuous unrealized loss position, Fair Value	4,424,145	4,672,612
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(538,219)	(511,864)
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	0	3,477
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	(7)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	271,088	477,306
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(4,595)	(14,738)
Available-for-sale securities, continuous unrealized loss position, Fair Value	271,088	480,783
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(4,595)	(14,745)
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	56,777	11,855
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(1,224)	(34)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,361,628	1,427,975
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(137,618)	(125,148)
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,418,405	1,439,830
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(138,842)	(125,182)
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	198,385	1,631
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(4,990)	(1)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,048,727	2,361,089
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(373,756)	(352,882)
Available-for-sale securities, continuous unrealized loss position, Fair Value	2,247,112	2,362,720
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(378,746)	(352,883)
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	196,640	0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(2,249)	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	190,189	284,324
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(7,409)	(11,339)
Available-for-sale securities, continuous unrealized loss position, Fair Value	386,829	284,324
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(9,658)	(11,339)
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	4,871	0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(44)	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	95,840	104,955
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(6,334)	(7,715)
Available-for-sale securities, continuous unrealized loss position, Fair Value	100,711	104,955
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ (6,378)	$ (7,715)