Investment Securities - Unrealized Losses and Estimated Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	$ 116,466	$ 143,467
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(3,506)	(2,794)
Fair Value of Available-for-Sale Securities, 12 Months or More	2,878,261	2,887,122
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(361,287)	(333,485)
Fair Value of Available-for-Sale Securities, Total	2,994,727	3,030,589
Unrealized Losses of Available-for-Sale Securities, Total	(364,793)	(336,279)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	52,941	69,991
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(1,440)	(1,158)
Fair Value of Held-to-Maturity Securities, 12 Months or More	1,082,683	1,062,828
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(120,217)	(110,905)
Fair Value of Held-to-Matuirty Securities, Total	1,135,624	1,132,819
Unrealized Losses of Held-to-Maturity Securities	(121,657)	(112,063)
U.S. government-sponsored enterprises
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	29,035	2,742
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(529)	(2)
Fair Value of Available-for-Sale Securities, 12 Months or More	177,346	180,569
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(18,008)	(17,091)
Fair Value of Available-for-Sale Securities, Total	206,381	183,311
Unrealized Losses of Available-for-Sale Securities, Total	(18,537)	(17,093)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	0	0
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	0	0
Fair Value of Held-to-Maturity Securities, 12 Months or More	40,011	40,677
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(3,342)	(2,607)
Fair Value of Held-to-Matuirty Securities, Total	40,011	40,677
Unrealized Losses of Held-to-Maturity Securities	(3,342)	(2,607)
U.S. government-sponsored mortgage-backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	34,770	102,831
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(956)	(2,166)
Fair Value of Available-for-Sale Securities, 12 Months or More	1,365,034	1,392,318
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(214,231)	(189,391)
Fair Value of Available-for-Sale Securities, Total	1,399,804	1,495,149
Unrealized Losses of Available-for-Sale Securities, Total	(215,187)	(191,557)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	42,125	48,498
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(960)	(861)
Fair Value of Held-to-Maturity Securities, 12 Months or More	80,859	65,573
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(5,133)	(3,501)
Fair Value of Held-to-Matuirty Securities, Total	122,984	114,071
Unrealized Losses of Held-to-Maturity Securities	(6,093)	(4,362)
Private mortgage-backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	0	9,298
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	0	(226)
Fair Value of Available-for-Sale Securities, 12 Months or More	173,541	166,107
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(16,614)	(15,891)
Fair Value of Available-for-Sale Securities, Total	173,541	175,405
Unrealized Losses of Available-for-Sale Securities, Total	(16,614)	(16,117)
Non-government-sponsored asset backed securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	17,486	0
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(15)	0
Fair Value of Available-for-Sale Securities, 12 Months or More	212,754	213,838
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(6,406)	(7,551)
Fair Value of Available-for-Sale Securities, Total	230,240	213,838
Unrealized Losses of Available-for-Sale Securities, Total	(6,421)	(7,551)
State and political subdivisions
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	32,760	28,596
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(921)	(400)
Fair Value of Available-for-Sale Securities, 12 Months or More	782,538	769,860
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(80,726)	(75,531)
Fair Value of Available-for-Sale Securities, Total	815,298	798,456
Unrealized Losses of Available-for-Sale Securities, Total	(81,647)	(75,931)
Held-to-maturity:
Fair Value of Held-to-Maturity Securities, Less Than 12 Months	10,816	21,493
Unrealized Losses of Held-to-Maturity Securities, Less Than 12 Months	(480)	(297)
Fair Value of Held-to-Maturity Securities, 12 Months or More	961,813	956,578
Unrealized Losses of Held-to-Maturity Securities, 12 Months or More	(111,742)	(104,797)
Fair Value of Held-to-Matuirty Securities, Total	972,629	978,071
Unrealized Losses of Held-to-Maturity Securities	(112,222)	(105,094)
Other securities
Available-for-sale:
Fair Value of Available-for-Sale Securities, Less Than 12 Months	2,415	0
Unrealized Losses of Available-for-Sale Securities, Less Than 12 Months	(1,085)	0
Fair Value of Available-for-Sale Securities, 12 Months or More	167,048	164,430
Unrealized Losses of Available-for-Sale Securities, 12 Months or More	(25,302)	(28,030)
Fair Value of Available-for-Sale Securities, Total	169,463	164,430
Unrealized Losses of Available-for-Sale Securities, Total	$ (26,387)	$ (28,030)

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