Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 267,067	$ 456,673	$ 122,818
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(4,254)	(8,507)	(936)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	3,880,759	3,967,472	4,469,213
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(494,886)	(529,712)	(558,749)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	4,147,826	4,424,145	4,592,031
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(499,140)	(538,219)	(559,685)
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	0	2,438
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	0	(16)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	211,193	271,088	429,730
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(2,979)	(4,595)	(13,215)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	211,193	271,088	432,168
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(2,979)	(4,595)	(13,231)
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	72,501	56,777	33,073
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(2,111)	(1,224)	(114)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,329,121	1,361,628	1,398,108
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(158,526)	(137,618)	(138,944)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	1,401,622	1,418,405	1,431,181
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(160,637)	(138,842)	(139,058)
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	167,666	198,385	22,586
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(1,950)	(4,990)	(299)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,059,869	2,048,727	2,261,311
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(322,716)	(373,756)	(386,392)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	2,227,535	2,247,112	2,283,897
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(324,666)	(378,746)	(386,691)
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	16,992	196,640	64,721
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(179)	(2,249)	(507)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	188,744	190,189	275,924
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(5,518)	(7,409)	(11,837)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	205,736	386,829	340,645
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	(5,697)	(9,658)	(12,344)
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	9,908	4,871	0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	(14)	(44)	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	91,832	95,840	104,140
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	(5,147)	(6,334)	(8,361)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total	101,740	100,711	104,140
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ (5,161)	$ (6,378)	$ (8,361)