Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 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	$ 2,689,753
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	16,918
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	12,642,997
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,384,877
Available for Sale, Total Estimated Fair Value	15,332,750
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,401,795	$ 1,681,985
US 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	4,927,590
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	285,305
Available for Sale, Total Estimated Fair Value	4,927,590
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	285,305	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	879,380
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6,883
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,062,456
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	869,455
Available for Sale, Total Estimated Fair Value	5,941,836
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	876,338	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	1,810,373
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10,035
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,652,951
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	230,117
Available for Sale, Total Estimated Fair Value	4,463,324
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 240,152	$ 334,388