Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 3,259,440	$ 3,252,586
Available-for-Sale, Unrealized Gains	9,252	7,943
Available-for-Sale, Unrealized Losses	(87,027)	(64,322)
Available-for-Sale securities, Fair Value	3,181,665	3,196,207	$ 2,814,338
Held-to-Maturity, Amortized Cost	525,718	684,063	695,139
Held-to-Maturity, Unrealized Gains	7,267	43,648
Held-to-Maturity, Unrealized Losses	(8,388)	(8)
Held-to-Maturity, Fair Value	524,597	727,703	749,363
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	5,312	5,473
Available-for-Sale, Unrealized Gains		83
Available-for-Sale, Unrealized Losses	(55)	(5)
Available-for-Sale securities, Fair Value	5,257	5,551	5,634
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	583,644	675,643
Available-for-Sale, Unrealized Gains		3
Available-for-Sale, Unrealized Losses	(15,413)	(11,360)
Available-for-Sale securities, Fair Value	568,231	664,286	580,624
Held-to-Maturity, Amortized Cost	73,570
Held-to-Maturity, Unrealized Losses	(2,286)
Held-to-Maturity, Fair Value	71,284
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,503,636	1,704,014
Available-for-Sale, Unrealized Gains	501	1,600
Available-for-Sale, Unrealized Losses	(55,611)	(37,932)
Available-for-Sale securities, Fair Value	1,448,526	1,667,682	1,462,111
Held-to-Maturity, Amortized Cost	139,594	6,903
Held-to-Maturity, Unrealized Gains	58	153
Held-to-Maturity, Unrealized Losses	(3,868)
Held-to-Maturity, Fair Value	135,784	7,056
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	793,141	529,835
Available-for-Sale, Unrealized Gains	8,406	5,085
Available-for-Sale, Unrealized Losses	(4,014)	(4,727)
Available-for-Sale securities, Fair Value	797,533	530,193	$ 431,874
Held-to-Maturity, Amortized Cost	312,554	677,160
Held-to-Maturity, Unrealized Gains	7,209	43,495
Held-to-Maturity, Unrealized Losses	(2,234)	(8)
Held-to-Maturity, Fair Value	317,529	720,647
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,887	16,605
Available-for-Sale, Unrealized Losses	(5,682)	(8,157)
Available-for-Sale securities, Fair Value	8,205	8,448
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	359,820	321,016
Available-for-Sale, Unrealized Gains	345	1,172
Available-for-Sale, Unrealized Losses	(6,252)	(2,141)
Available-for-Sale securities, Fair Value	$ 353,913	$ 320,047

Schedule of available for sale and held to maturity securities.

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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, before tax, of unrealized loss 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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Amount 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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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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