Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Amortized Cost	$ 3,882,541	$ 4,843,313
Gross Unrealized Gains	55,292	13,648
Gross Unrealized Losses	(4,473)	(73,382)
Estimated Fair Value	3,933,360	4,783,579
Schedule of Held-to-maturity Securities
Amortized Cost	182,961	207,446
Gross Unrealized Gains	7,177	339
Gross Unrealized Losses	(239)	(3,508)
Estimated Fair Value	189,899	204,277
Fair value of securities held as collateral	2,200,000	2,400,000
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Amortized Cost	39,916	995
Gross Unrealized Gains	281	3
Gross Unrealized Losses	0	0
Estimated Fair Value	40,197	998
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	160,873	177,566
Gross Unrealized Gains	7,607	2,045
Gross Unrealized Losses	0	(723)
Estimated Fair Value	168,480	178,888
Schedule of Held-to-maturity Securities
Amortized Cost	166,386	188,684
Gross Unrealized Gains	7,147	309
Gross Unrealized Losses	0	(2,497)
Estimated Fair Value	173,533	186,496
Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	2,876,069	3,837,584
Gross Unrealized Gains	27,423	8,886
Gross Unrealized Losses	(3,836)	(57,073)
Estimated Fair Value	2,899,656	3,789,397
Schedule of Held-to-maturity Securities
Amortized Cost	16,575	18,762
Gross Unrealized Gains	30	30
Gross Unrealized Losses	(239)	(1,011)
Estimated Fair Value	16,366	17,781
Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	704,661	730,148
Gross Unrealized Gains	17,202	2,363
Gross Unrealized Losses	(404)	(14,799)
Estimated Fair Value	721,459	717,712
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	101,022	97,020
Gross Unrealized Gains	2,779	351
Gross Unrealized Losses	(233)	(787)
Estimated Fair Value	$ 103,568	$ 96,584

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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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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