Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 391,054	$ 1,909,674	$ 5,136,450
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	13,114	118,033	472,656
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,576,738	3,378,251	327,839
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	608,383	561,193	43,356
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	4,967,792	5,287,925	5,464,289
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	621,497	679,226	516,012
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	47,498	307,012	466,438
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	1,614	11,650	14,752
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	436,278	173,105	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	22,734	14,087	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	483,776	480,117	466,438
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	24,348	25,737	14,752
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	137,728	770,469	1,623,146
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	1,838	55,943	178,446
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,457,431	946,571	24,038
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	164,796	150,856	3,919
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	1,595,159	1,717,040	1,647,184
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	166,634	206,799	182,365
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	68,439	470,970	2,631,511
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	3,609	37,065	266,352
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,403,837	2,143,869	277,404
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	399,151	380,497	34,797
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	2,472,276	2,614,839	2,908,915
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	402,760	417,562	301,149
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	99,347	319,303	341,997
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	4,771	11,677	8,819
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	215,040	54,862	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	12,671	6,369	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	314,387	374,165	341,997
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	17,442	18,046	8,819
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	38,042	41,920	73,358
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	1,282	1,698	4,287
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	64,152	59,844	26,397
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	9,031	9,384	4,640
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	102,194	101,764	99,755
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 10,313	$ 11,082	$ 8,927