Investment Securities - Gross Unrealized Losses on Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities Available for Sale
Less than 12 months, Fair Value	$ 145,286	$ 66,657
Less than 12 months, Unrealized Loss	(1,028)	(751)
12 months or more, Fair Value	1,803,056	1,997,383
12 months or more, Unrealized Loss	(247,448)	(231,595)
Total, Fair Value	1,948,342	2,064,040
Total, Unrealized Loss	(248,476)	(232,346)
Debt Securities Held to Maturity
Less than 12 months, Fair Value	1,618	0
Less than 12 Months, Unrealized Loss	(8)	0
12 months or more, Fair Value	116,678	123,271
12 months or more, Unrealized Loss	(9,276)	(8,374)
Total, Fair Value	118,296	123,271
Total, Unrealized Loss	(9,284)	(8,374)
Obligations of U.S. government agencies
Debt Securities Available for Sale
Less than 12 months, Fair Value	442	224
Less than 12 months, Unrealized Loss	(3)	0
12 months or more, Fair Value	1,169,687	1,221,320
12 months or more, Unrealized Loss	(179,485)	(165,037)
Total, Fair Value	1,170,129	1,221,544
Total, Unrealized Loss	(179,488)	(165,037)
Debt Securities Held to Maturity
Less than 12 months, Fair Value	0	0
Less than 12 Months, Unrealized Loss	0	0
12 months or more, Fair Value	115,660	122,259
12 months or more, Unrealized Loss	(9,239)	(8,331)
Total, Fair Value	115,660	122,259
Total, Unrealized Loss	(9,239)	(8,331)
Obligations of states and political subdivisions
Debt Securities Available for Sale
Less than 12 months, Fair Value	6,760	6,229
Less than 12 months, Unrealized Loss	(152)	(75)
12 months or more, Fair Value	219,908	216,497
12 months or more, Unrealized Loss	(28,580)	(26,697)
Total, Fair Value	226,668	222,726
Total, Unrealized Loss	(28,732)	(26,772)
Debt Securities Held to Maturity
Less than 12 months, Fair Value	1,618	0
Less than 12 Months, Unrealized Loss	(8)	0
12 months or more, Fair Value	1,018	1,012
12 months or more, Unrealized Loss	(37)	(43)
Total, Fair Value	2,636	1,012
Total, Unrealized Loss	(45)	(43)
Corporate bonds
Debt Securities Available for Sale
Less than 12 months, Fair Value	0	0
Less than 12 months, Unrealized Loss	0	0
12 months or more, Fair Value	5,746	5,602
12 months or more, Unrealized Loss	(429)	(571)
Total, Fair Value	5,746	5,602
Total, Unrealized Loss	(429)	(571)
Asset backed securities
Debt Securities Available for Sale
Less than 12 months, Fair Value	94,861	15,928
Less than 12 months, Unrealized Loss	(350)	(93)
12 months or more, Fair Value	146,838	264,731
12 months or more, Unrealized Loss	(3,477)	(4,095)
Total, Fair Value	241,699	280,659
Total, Unrealized Loss	(3,827)	(4,188)
Non-agency collateralized mortgage obligations
Debt Securities Available for Sale
Less than 12 months, Fair Value	43,223	44,276
Less than 12 months, Unrealized Loss	(523)	(583)
12 months or more, Fair Value	260,877	289,233
12 months or more, Unrealized Loss	(35,477)	(35,195)
Total, Fair Value	304,100	333,509
Total, Unrealized Loss	$ (36,000)	$ (35,778)