Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,840,927	$ 4,445,620
Allowance for Credit Losses	(2,525)	(842)	$ (842)	$ (842)
Net Carrying Amount	3,838,402	4,444,778
Gross Unrealized Gains	5,718	4,779
Gross Unrealized (Losses)	(336,279)	(407,967)
Estimated Fair Value	3,507,841	4,041,590
U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	361,494	682,316
Allowance for Credit Losses	0	0
Net Carrying Amount	361,494	682,316
Gross Unrealized Gains	2,247	2,713
Gross Unrealized (Losses)	(17,093)	(23,209)
Estimated Fair Value	346,648	661,820
U.S. government-sponsored mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,711,668	1,900,796
Allowance for Credit Losses	0	0
Net Carrying Amount	1,711,668	1,900,796
Gross Unrealized Gains	310	71
Gross Unrealized (Losses)	(191,557)	(215,405)
Estimated Fair Value	1,520,421	1,685,462
Private mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	191,522	197,435
Allowance for Credit Losses	0	0
Net Carrying Amount	191,522	197,435
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(16,117)	(18,302)
Estimated Fair Value	175,405	179,133
Non-government-sponsored asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	370,203	428,933
Allowance for Credit Losses	0	0
Net Carrying Amount	370,203	428,933
Gross Unrealized Gains	821	95
Gross Unrealized (Losses)	(7,551)	(14,654)
Estimated Fair Value	363,473	414,374
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	990,318	1,021,188
Allowance for Credit Losses	0	(842)
Net Carrying Amount	990,318	1,020,346
Gross Unrealized Gains	1,938	1,649
Gross Unrealized (Losses)	(75,931)	(115,698)
Estimated Fair Value	916,325	906,297
Other securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	215,722	214,952
Allowance for Credit Losses	(2,525)	0
Net Carrying Amount	213,197	214,952
Gross Unrealized Gains	402	251
Gross Unrealized (Losses)	(28,030)	(20,699)
Estimated Fair Value	$ 185,569	$ 194,504

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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