Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 4,158,116
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17,365
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	8,506,358
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,121,126
Debt Securities, Available-for-sale, Unrealized Loss Position	12,664,474
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,138,491	$ 1,126,817
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	795,502
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	168
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,130,449
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	154,061
Debt Securities, Available-for-sale, Unrealized Loss Position	2,925,951
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	154,229	148,357
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	2,922,925
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	15,554
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,019,978
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	765,946
Debt Securities, Available-for-sale, Unrealized Loss Position	6,942,903
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	781,500	751,013
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	439,689
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,355,931
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	201,119
Debt Securities, Available-for-sale, Unrealized Loss Position	2,795,620
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 202,762	$ 227,447