Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Accrued interest on available-for-sale and held-to-maturity securities	$ 13,300	$ 15,100
Schedule of Available-for-sale Securities
Amortized Cost	3,790,375	3,882,541
Gross Unrealized Gains	124,908	55,292
Gross Unrealized Losses	(323)	(4,473)
Estimated Fair Value	3,914,960	3,933,360
Schedule of Held-to-maturity Securities
Amortized Cost	177,960	182,961
Gross Unrealized Gains	8,443	7,177
Gross Unrealized Losses	(7)	(239)
Estimated Fair Value	186,396	189,899
Fair value of securities held as collateral	2,400,000	2,200,000
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Amortized Cost	39,944	39,916
Gross Unrealized Gains	425	281
Gross Unrealized Losses	0	0
Estimated Fair Value	40,369	40,197
Schedule of Held-to-maturity Securities
Amortized Cost	15,600
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	158,578	160,873
Gross Unrealized Gains	6,984	7,607
Gross Unrealized Losses	0	0
Estimated Fair Value	165,562	168,480
Schedule of Held-to-maturity Securities
Amortized Cost	162,330	166,386
Gross Unrealized Gains	8,233	7,147
Gross Unrealized Losses	0	0
Estimated Fair Value	170,563	173,533
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	2,797,087	2,876,069
Gross Unrealized Gains	82,191	27,423
Gross Unrealized Losses	(70)	(3,836)
Estimated Fair Value	2,879,208	2,899,656
Schedule of Held-to-maturity Securities
Amortized Cost	15,630	16,575
Gross Unrealized Gains	210	30
Gross Unrealized Losses	(7)	(239)
Estimated Fair Value	15,833	16,366
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	681,145	704,661
Gross Unrealized Gains	32,078	17,202
Gross Unrealized Losses	(13)	(404)
Estimated Fair Value	713,210	721,459
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	113,621	101,022
Gross Unrealized Gains	3,230	2,779
Gross Unrealized Losses	(240)	(233)
Estimated Fair Value	$ 116,611	$ 103,568

Debt Securities, Available-For-Sale And Held-To-Maturity, Accrued Interest, After Allowance For Credit Loss

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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 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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