Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,881,429	$ 4,843,313
Gross Unrealized Gains	30,374	13,648
Gross Unrealized Losses	(38,025)	(73,382)
Estimated Fair Value	4,873,778	4,783,579
Schedule of Held-to-maturity Securities
Amortized Cost	198,958	207,446
Gross Unrealized Gains	2,394	339
Gross Unrealized Losses	(784)	(3,508)
Estimated Fair Value	200,568	204,277
Fair value of securities held as collateral	2,300,000	2,400,000
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Amortized Cost	40,822	995
Gross Unrealized Gains	130	3
Gross Unrealized Losses	0	0
Estimated Fair Value	40,952	998
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	175,271	177,566
Gross Unrealized Gains	4,740	2,045
Gross Unrealized Losses	(32)	(723)
Estimated Fair Value	179,979	178,888
Schedule of Held-to-maturity Securities
Amortized Cost	180,472	188,684
Gross Unrealized Gains	2,365	309
Gross Unrealized Losses	(42)	(2,497)
Estimated Fair Value	182,795	186,496
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	3,757,469	3,837,584
Gross Unrealized Gains	16,972	8,886
Gross Unrealized Losses	(29,763)	(57,073)
Estimated Fair Value	3,744,678	3,789,397
Schedule of Held-to-maturity Securities
Amortized Cost	18,486	18,762
Gross Unrealized Gains	29	30
Gross Unrealized Losses	(742)	(1,011)
Estimated Fair Value	17,773	17,781
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	779,487	730,148
Gross Unrealized Gains	6,656	2,363
Gross Unrealized Losses	(7,716)	(14,799)
Estimated Fair Value	778,427	717,712
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	128,380	97,020
Gross Unrealized Gains	1,876	351
Gross Unrealized Losses	(514)	(787)
Estimated Fair Value	$ 129,742	$ 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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