Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,808,975	$ 4,647,931
Gross Unrealized Gains	767	5,972
Gross Unrealized Losses	(175,618)	(63,841)
Estimated Fair Value	4,634,124	4,590,062
Schedule of Held-to-maturity Securities
Amortized Cost	213,561	227,318
Gross Unrealized Gains	377	1,571
Gross Unrealized Losses	(6,629)	(925)
Estimated Fair Value	207,309	227,964
Fair value of securities held as collateral	2,300,000	2,100,000
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Amortized Cost	40,996	41,003
Gross Unrealized Gains	5	18
Gross Unrealized Losses	(740)	(406)
Estimated Fair Value	40,261	40,615
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	265,565	271,451
Gross Unrealized Gains	694	4,246
Gross Unrealized Losses	(4,852)	(1,493)
Estimated Fair Value	261,407	274,204
Schedule of Held-to-maturity Securities
Amortized Cost	194,382	206,736
Gross Unrealized Gains	349	1,530
Gross Unrealized Losses	(5,277)	(275)
Estimated Fair Value	189,454	207,991
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Amortized Cost	3,654,417	3,675,367
Gross Unrealized Gains	68	1,233
Gross Unrealized Losses	(140,816)	(52,090)
Estimated Fair Value	3,513,669	3,624,510
Schedule of Held-to-maturity Securities
Amortized Cost	19,179	20,582
Gross Unrealized Gains	28	41
Gross Unrealized Losses	(1,352)	(650)
Estimated Fair Value	17,855	19,973
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Amortized Cost	721,899	546,105
Gross Unrealized Gains	0	228
Gross Unrealized Losses	(26,513)	(8,938)
Estimated Fair Value	695,386	537,395
Other securities
Schedule of Available-for-sale Securities
Amortized Cost	126,098	114,005
Gross Unrealized Gains	0	247
Gross Unrealized Losses	(2,697)	(914)
Estimated Fair Value	$ 123,401	$ 113,338

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 of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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