Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,410,272	$ 3,840,927
Allowance for Credit Losses	(2,195)	(2,525)	$ (842)	$ (842)
Net Carrying Amount	3,408,077	3,838,402
Gross Unrealized Gains	3,166	5,718
Gross Unrealized (Losses)	(338,604)	(336,279)
Estimated Fair Value	3,072,639	3,507,841
U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	297,698	361,494
Allowance for Credit Losses	0	0
Net Carrying Amount	297,698	361,494
Gross Unrealized Gains	1,164	2,247
Gross Unrealized (Losses)	(14,072)	(17,093)
Estimated Fair Value	284,790	346,648
U.S. government-sponsored mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,527,463	1,711,668
Allowance for Credit Losses	0	0
Net Carrying Amount	1,527,463	1,711,668
Gross Unrealized Gains	760	310
Gross Unrealized (Losses)	(203,539)	(191,557)
Estimated Fair Value	1,324,684	1,520,421
Private mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	184,643	191,522
Allowance for Credit Losses	0	0
Net Carrying Amount	184,643	191,522
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(13,249)	(16,117)
Estimated Fair Value	171,394	175,405
Non-government-sponsored asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	228,751	370,203
Allowance for Credit Losses	0	0
Net Carrying Amount	228,751	370,203
Gross Unrealized Gains	331	821
Gross Unrealized (Losses)	(3,434)	(7,551)
Estimated Fair Value	225,648	363,473
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	956,055	990,318
Allowance for Credit Losses	0	0
Net Carrying Amount	956,055	990,318
Gross Unrealized Gains	335	1,938
Gross Unrealized (Losses)	(86,029)	(75,931)
Estimated Fair Value	870,361	916,325
Other securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	215,662	215,722
Allowance for Credit Losses	(2,195)	(2,525)
Net Carrying Amount	213,467	213,197
Gross Unrealized Gains	576	402
Gross Unrealized (Losses)	(18,281)	(28,030)
Estimated Fair Value	$ 195,762	$ 185,569

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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