Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 3,777,092
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	29,522
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	8,704,253
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,167,039
Debt Securities, Available-for-sale, Unrealized Loss Position	12,481,345
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,196,561	$ 1,126,817
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,037,165
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	235
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,254,388
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	153,192
Debt Securities, Available-for-sale, Unrealized Loss Position	3,291,553
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	153,427	148,357
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,381,582
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12,322
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,121,310
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	764,460
Debt Securities, Available-for-sale, Unrealized Loss Position	5,502,892
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	776,782	751,013
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,358,345
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	16,965
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,328,555
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	249,387
Debt Securities, Available-for-sale, Unrealized Loss Position	3,686,900
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 266,352	$ 227,447