Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,402,001	$ 4,843,313
Gross Unrealized Gains	60,870	13,648
Gross Unrealized Losses	(7,563)	(73,382)
Estimated Fair Value	4,455,308	4,783,579
Schedule of Held-to-maturity Securities
Amortized Cost	192,917	207,446
Gross Unrealized Gains	5,437	339
Gross Unrealized Losses	(342)	(3,508)
Estimated Fair Value	198,012	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	39,859	995
Gross Unrealized Gains	341	3
Gross Unrealized Losses	0	0
Estimated Fair Value	40,200	998
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	172,160	177,566
Gross Unrealized Gains	6,698	2,045
Gross Unrealized Losses	0	(723)
Estimated Fair Value	178,858	178,888
Schedule of Held-to-maturity Securities
Amortized Cost	175,281	188,684
Gross Unrealized Gains	5,407	309
Gross Unrealized Losses	0	(2,497)
Estimated Fair Value	180,688	186,496
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	3,341,602	3,837,584
Gross Unrealized Gains	32,909	8,886
Gross Unrealized Losses	(6,364)	(57,073)
Estimated Fair Value	3,368,147	3,789,397
Schedule of Held-to-maturity Securities
Amortized Cost	17,636	18,762
Gross Unrealized Gains	30	30
Gross Unrealized Losses	(342)	(1,011)
Estimated Fair Value	17,324	17,781
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	742,760	730,148
Gross Unrealized Gains	17,383	2,363
Gross Unrealized Losses	(1,107)	(14,799)
Estimated Fair Value	759,036	717,712
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	105,620	97,020
Gross Unrealized Gains	3,539	351
Gross Unrealized Losses	(92)	(787)
Estimated Fair Value	$ 109,067	$ 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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