Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 3,267,559
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38,301
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	51,638
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,764
Available for Sale, Total Estimated Fair Value	3,319,197
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	40,065	$ 4
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	684,242
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,602
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available for Sale, Total Estimated Fair Value	684,242
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9,602	0
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	2,219,261
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23,625
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	45,963
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,464
Available for Sale, Total Estimated Fair Value	2,265,224
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	25,089	4
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	364,056
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5,074
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,675
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	300
Available for Sale, Total Estimated Fair Value	369,731
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 5,374	$ 0