Securities - Securities Available for Sale, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 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	$ 9,480,456
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	477,162
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	7,355,219
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,530,871
Debt Securities, Available-for-sale, Unrealized Loss Position	16,835,675
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,008,033	$ 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	538,419
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6,137
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,845,095
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	174,726
Debt Securities, Available-for-sale, Unrealized Loss Position	2,383,514
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	180,863	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	5,856,558
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	267,168
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,676,684
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,028,308
Debt Securities, Available-for-sale, Unrealized Loss Position	9,533,242
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,295,476	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	3,085,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	203,857
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,833,440
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	327,837
Debt Securities, Available-for-sale, Unrealized Loss Position	4,918,919
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 531,694	$ 227,447