Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 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	$ 326,569
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	921
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	12,715,142
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,160,128
Debt Securities, Available-for-sale, Unrealized Loss Position	13,041,711
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,161,049	$ 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	3,987,746
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	200,877
Debt Securities, Available-for-sale, Unrealized Loss Position	3,987,746
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	200,877	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	719
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,093,851
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	748,155
Debt Securities, Available-for-sale, Unrealized Loss Position	5,094,570
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	748,157	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	325,850
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	919
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,633,545
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	211,096
Debt Securities, Available-for-sale, Unrealized Loss Position	3,959,395
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 212,015	$ 240,152