Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 4,789,430	$ 2,474,964	$ 1,155,527
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	265,748	21,474	18,355
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	357,123	89,767	999
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	29,415	2,237	10
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	5,146,553	2,564,731	1,156,526
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	295,163	23,711	18,365
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	301,344
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	10,516
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	301,344
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	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	1,518,732	163,698	533,213
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	85,015	1,096	8,198
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	25,679	18,943
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	2,440	122
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	1,544,411	182,641	533,213
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	87,455	1,218	8,198
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	2,725,934	2,263,010	589,353
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	163,034	19,742	8,618
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	303,344	54,392	999
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	23,920	1,544	10
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	3,029,278	2,317,402	590,352
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	186,954	21,286	8,628
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	202,719	820	471
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	4,752	1
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	202,719	820	471
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	4,752	1
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	40,701	47,436	32,490
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	2,431	635	1,539
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	28,100	16,432
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3,055	571
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	68,801	63,868	32,490
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 5,486	$ 1,206	$ 1,539