Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 4,172,617	$ 3,252,586
Available-for-Sale, Unrealized Gains	21,372	7,943
Available-for-Sale, Unrealized Losses	(70,573)	(64,322)
Available-for-Sale securities, Fair Value	4,123,416	3,196,207
Held-to-Maturity, Amortized Cost	506,334	684,063
Held-to-Maturity, Unrealized Gains	6,247	43,648
Held-to-Maturity, Unrealized Losses	(6,478)	(8)
Held-to-Maturity, Fair Value	506,103	727,703
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	5,332	5,473
Available-for-Sale, Unrealized Gains		83
Available-for-Sale, Unrealized Losses	(31)	(5)
Available-for-Sale securities, Fair Value	5,301	5,551
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	639,458	675,643
Available-for-Sale, Unrealized Gains	35	3
Available-for-Sale, Unrealized Losses	(11,342)	(11,360)
Available-for-Sale securities, Fair Value	628,151	664,286
Held-to-Maturity, Amortized Cost	73,986
Held-to-Maturity, Unrealized Losses	(1,627)
Held-to-Maturity, Fair Value	72,359
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	2,243,774	1,704,014
Available-for-Sale, Unrealized Gains	9,738	1,600
Available-for-Sale, Unrealized Losses	(44,217)	(37,932)
Available-for-Sale securities, Fair Value	2,209,295	1,667,682
Held-to-Maturity, Amortized Cost	127,120	6,903
Held-to-Maturity, Unrealized Gains	39	153
Held-to-Maturity, Unrealized Losses	(2,750)
Held-to-Maturity, Fair Value	124,409	7,056
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	932,757	529,835
Available-for-Sale, Unrealized Gains	11,113	5,085
Available-for-Sale, Unrealized Losses	(3,441)	(4,727)
Available-for-Sale securities, Fair Value	940,429	530,193
Held-to-Maturity, Amortized Cost	305,228	677,160
Held-to-Maturity, Unrealized Gains	6,208	43,495
Held-to-Maturity, Unrealized Losses	(2,101)	(8)
Held-to-Maturity, Fair Value	309,335	720,647
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,861	16,605
Available-for-Sale, Unrealized Losses	(5,366)	(8,157)
Available-for-Sale securities, Fair Value	8,495	8,448
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	337,435	321,016
Available-for-Sale, Unrealized Gains	486	1,172
Available-for-Sale, Unrealized Losses	(6,176)	(2,141)
Available-for-Sale securities, Fair Value	$ 331,745	$ 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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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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