Investment Securities - Amortized Cost and Fair Value of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,763,609	$ 3,840,927
Allowance for Credit Losses	(2,525)	(2,525)	$ (842)	$ (842)
Net Carrying Amount	3,761,084	3,838,402
Gross Unrealized Gains	4,593	5,718
Gross Unrealized (Losses)	(364,793)	(336,279)
Estimated Fair Value	3,400,884	3,507,841
U.S. government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	342,771	361,494
Allowance for Credit Losses	0	0
Net Carrying Amount	342,771	361,494
Gross Unrealized Gains	1,441	2,247
Gross Unrealized (Losses)	(18,537)	(17,093)
Estimated Fair Value	325,675	346,648
U.S. government-sponsored mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,667,526	1,711,668
Allowance for Credit Losses	0	0
Net Carrying Amount	1,667,526	1,711,668
Gross Unrealized Gains	396	310
Gross Unrealized (Losses)	(215,187)	(191,557)
Estimated Fair Value	1,452,735	1,520,421
Private mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	190,155	191,522
Allowance for Credit Losses	0	0
Net Carrying Amount	190,155	191,522
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(16,614)	(16,117)
Estimated Fair Value	173,541	175,405
Non-government-sponsored asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	367,066	370,203
Allowance for Credit Losses	0
Net Carrying Amount	367,066	370,203
Gross Unrealized Gains	695	821
Gross Unrealized (Losses)	(6,421)	(7,551)
Estimated Fair Value	361,340	363,473
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	980,415	990,318
Allowance for Credit Losses	0	0
Net Carrying Amount	980,415	990,318
Gross Unrealized Gains	1,104	1,938
Gross Unrealized (Losses)	(81,647)	(75,931)
Estimated Fair Value	899,872	916,325
Other securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	215,676	215,722
Allowance for Credit Losses	(2,525)	(2,525)
Net Carrying Amount	213,151	213,197
Gross Unrealized Gains	957	402
Gross Unrealized (Losses)	(26,387)	(28,030)
Estimated Fair Value	$ 187,721	$ 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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