Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 3,734,243
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	147,460
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	12,325,693
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,064,608
Debt Securities, Available-for-sale, Unrealized Loss Position	16,059,936
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,212,068	$ 1,681,985
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	5,043,152
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	416,606
Debt Securities, Available-for-sale, Unrealized Loss Position	5,043,152
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	416,606	398,959
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	1,486,570
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	67,981
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,806,198
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,214,929
Debt Securities, Available-for-sale, Unrealized Loss Position	6,292,768
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,282,910	948,638
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	2,247,673
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	79,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,476,343
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	433,073
Debt Securities, Available-for-sale, Unrealized Loss Position	4,724,016
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 512,552	$ 334,388