Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Jun. 30, 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	$ 1,191,218
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	13,036
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	13,319,570
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,620,273
Debt Securities, Available-for-sale, Unrealized Loss Position	14,510,788
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,633,309	$ 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	3,882,331
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	302,607
Debt Securities, Available-for-sale, Unrealized Loss Position	3,882,331
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	302,607	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	508,001
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5,469
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,561,705
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,027,739
Debt Securities, Available-for-sale, Unrealized Loss Position	6,069,706
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,033,208	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	683,217
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7,567
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,875,534
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	289,927
Debt Securities, Available-for-sale, Unrealized Loss Position	4,558,751
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 297,494	$ 240,152