Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 122,818	$ 16,963	$ 616,799
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	936	42	13,693
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,469,213	4,655,649	4,499,790
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	558,749	511,822	568,401
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	4,592,031	4,672,612	5,116,589
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	559,685	511,864	582,094
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	2,438	3,477	191,725
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	16	7	4,193
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	429,730	477,306	292,587
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	13,215	14,738	16,289
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	432,168	480,783	484,312
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	13,231	14,745	20,482
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	33,073	11,855	126,532
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	114	34	1,448
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,398,108	1,427,975	1,470,950
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	138,944	125,148	161,903
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	1,431,181	1,439,830	1,597,482
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	139,058	125,182	163,351
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	22,586	1,631	91,738
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	299	1	2,937
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,261,311	2,361,089	2,489,524
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	386,392	352,882	371,224
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	2,283,897	2,362,720	2,581,262
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	386,691	352,883	374,161
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	64,721	0	164,301
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	507	0	3,948
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	275,924	284,324	185,635
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11,837	11,339	11,072
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	340,645	284,324	349,936
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	12,344	11,339	15,020
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	0	42,503
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss, Total	0	0	1,167
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	104,140	104,955	61,094
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	8,361	7,715	7,913
Debt Securities, Available-for-sale, Unrealized Loss Position, Total	104,140	104,955	103,597
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 8,361	$ 7,715	$ 9,080