Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 1,156,655
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10,509
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	13,246,908
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,582,642
Debt Securities, Available-for-sale, Unrealized Loss Position	14,403,563
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,593,151	$ 1,401,795
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	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,872,572
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	308,729
Debt Securities, Available-for-sale, Unrealized Loss Position	3,872,572
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	308,729	285,305
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	376,347
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3,974
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,633,391
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,000,392
Debt Securities, Available-for-sale, Unrealized Loss Position	6,009,738
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,004,366	876,338
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	780,308
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6,535
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,740,945
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	273,521
Debt Securities, Available-for-sale, Unrealized Loss Position	4,521,253
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 280,056	$ 240,152