Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,175,533	$ 4,843,313
Gross Unrealized Gains	66,763	13,648
Gross Unrealized Losses	(4,214)	(73,382)
Estimated Fair Value	4,238,082	4,783,579
Schedule of Held-to-maturity Securities
Amortized Cost	185,007	207,446
Gross Unrealized Gains	7,775	339
Gross Unrealized Losses	(196)	(3,508)
Estimated Fair Value	192,586	204,277
Fair value of securities held as collateral	2,100,000	2,400,000
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Amortized Cost	39,887	995
Gross Unrealized Gains	294	3
Gross Unrealized Losses	0	0
Estimated Fair Value	40,181	998
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	166,034	177,566
Gross Unrealized Gains	7,333	2,045
Gross Unrealized Losses	0	(723)
Estimated Fair Value	173,367	178,888
Schedule of Held-to-maturity Securities
Amortized Cost	167,888	188,684
Gross Unrealized Gains	7,725	309
Gross Unrealized Losses	0	(2,497)
Estimated Fair Value	175,613	186,496
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	3,135,619	3,837,584
Gross Unrealized Gains	31,354	8,886
Gross Unrealized Losses	(3,930)	(57,073)
Estimated Fair Value	3,163,043	3,789,397
Schedule of Held-to-maturity Securities
Amortized Cost	17,119	18,762
Gross Unrealized Gains	50	30
Gross Unrealized Losses	(196)	(1,011)
Estimated Fair Value	16,973	17,781
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	732,669	730,148
Gross Unrealized Gains	24,239	2,363
Gross Unrealized Losses	(179)	(14,799)
Estimated Fair Value	756,729	717,712
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	101,324	97,020
Gross Unrealized Gains	3,543	351
Gross Unrealized Losses	(105)	(787)
Estimated Fair Value	$ 104,762	$ 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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