Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 138,456	$ 16,963	$ 62,293
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	410	42	2,810
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	4,198,372	4,655,649	4,581,616
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	421,788	511,822	807,959
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	4,336,828	4,672,612	4,643,909
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	422,198	511,864	810,769
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	0	3,477	4,867
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	7	49
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	290,535	477,306	480,580
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	5,671	14,738	22,053
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	290,535	480,783	485,447
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	5,671	14,745	22,102
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	6,899	11,855	53,029
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	43	34	2,568
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,395,597	1,427,975	1,402,097
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	115,473	125,148	260,745
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	1,402,496	1,439,830	1,455,126
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	115,516	125,182	263,313
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	124,439	1,631	3,123
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	335	1	106
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,205,425	2,361,089	2,294,112
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	289,695	352,882	495,472
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	2,329,864	2,362,720	2,297,235
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	290,030	352,883	495,578
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	7,118	0	1,274
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	32	0	87
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	209,632	284,324	304,074
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	5,763	11,339	18,104
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	216,750	284,324	305,348
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	5,795	11,339	18,191
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	0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	0	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	97,183	104,955	100,753
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	5,186	7,715	11,585
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	97,183	104,955	100,753
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 5,186	$ 7,715	$ 11,585