Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 62,293	$ 1,909,674	$ 4,258,183
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	2,810	118,033	537,139
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,581,616	3,378,251	1,367,993
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	807,959	561,193	264,420
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	4,643,909	5,287,925	5,626,176
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	810,769	679,226	801,559
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	4,867	307,012	476,696
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	49	11,650	27,283
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	480,580	173,105	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	22,053	14,087	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	485,447	480,117	476,696
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	22,102	25,737	27,283
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	53,029	770,469	1,828,105
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	2,568	55,943	254,212
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,402,097	946,571	139,732
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	260,745	150,856	31,095
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	1,455,126	1,717,040	1,967,837
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	263,313	206,799	285,307
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	3,123	470,970	1,507,121
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	106	37,065	231,915
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,294,112	2,143,869	1,187,815
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	495,472	380,497	225,082
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	2,297,235	2,614,839	2,694,936
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	495,578	417,562	456,997
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	1,274	319,303	386,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	87	11,677	19,185
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	304,074	54,862	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	18,104	6,369	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	305,348	374,165	386,479
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	18,191	18,046	19,185
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	0	41,920	59,782
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	0	1,698	4,544
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	100,753	59,844	40,446
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11,585	9,384	8,243
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	100,753	101,764	100,228
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 11,585	$ 11,082	$ 12,787