Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 616,799	$ 1,909,674	$ 4,789,430
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	13,693	118,033	265,748
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,499,790	3,378,251	357,123
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	568,401	561,193	29,415
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	5,116,589	5,287,925	5,146,553
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	582,094	679,226	295,163
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	191,725	307,012	301,344
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	4,193	11,650	10,516
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	292,587	173,105	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	16,289	14,087	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	484,312	480,117	301,344
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	20,482	25,737	10,516
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	126,532	770,469	1,518,732
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	1,448	55,943	85,015
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,470,950	946,571	25,679
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	161,903	150,856	2,440
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	1,597,482	1,717,040	1,544,411
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	163,351	206,799	87,455
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	91,738	470,970	2,725,934
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	2,937	37,065	163,034
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,489,524	2,143,869	303,344
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	371,224	380,497	23,920
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	2,581,262	2,614,839	3,029,278
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	374,161	417,562	186,954
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	164,301	319,303	202,719
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	3,948	11,677	4,752
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	185,635	54,862	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11,072	6,369	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	349,936	374,165	202,719
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	15,020	18,046	4,752
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	42,503	41,920	40,701
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	1,167	1,698	2,431
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	61,094	59,844	28,100
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	7,913	9,384	3,055
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	103,597	101,764	68,801
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 9,080	$ 11,082	$ 5,486