Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 4,556,605	$ 4,172,617
Available-for-Sale, Unrealized Gains	49,749	21,372
Available-for-Sale, Unrealized Losses	(36,582)	(70,573)
Available-for-Sale securities, Fair Value	4,569,772	4,123,416	$ 3,219,863
Held-to-Maturity, Amortized Cost	484,834	506,334	535,153
Held-to-Maturity, Unrealized Gains	9,276	6,247
Held-to-Maturity, Unrealized Losses	(233)	(6,478)
Investment securities - held-to-maturity, fair value	493,877	506,103	536,143
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	9,739	5,332
Available-for-Sale, Unrealized Gains	51
Available-for-Sale, Unrealized Losses	(13)	(31)
Available-for-Sale securities, Fair Value	9,777	5,301	9,295
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	703,398	639,458
Available-for-Sale, Unrealized Gains	172	35
Available-for-Sale, Unrealized Losses	(5,056)	(11,342)
Available-for-Sale securities, Fair Value	698,514	628,151	572,689
Held-to-Maturity, Amortized Cost	74,195	73,986
Held-to-Maturity, Unrealized Gains	30	0
Held-to-Maturity, Unrealized Losses	(39)	(1,627)
Investment securities - held-to-maturity, fair value	74,186	72,359
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	2,557,848	2,243,774
Available-for-Sale, Unrealized Gains	25,070	9,738
Available-for-Sale, Unrealized Losses	(22,215)	(44,217)
Available-for-Sale securities, Fair Value	2,560,703	2,209,295	1,477,896
Held-to-Maturity, Amortized Cost	123,627	127,120
Held-to-Maturity, Unrealized Gains	366	39
Held-to-Maturity, Unrealized Losses	(135)	(2,750)
Investment securities - held-to-maturity, fair value	123,858	124,409
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	944,149	932,757
Available-for-Sale, Unrealized Gains	21,883	11,113
Available-for-Sale, Unrealized Losses	(596)	(3,441)
Available-for-Sale securities, Fair Value	965,436	940,429	$ 843,488
Held-to-Maturity, Amortized Cost	287,012	305,228
Held-to-Maturity, Unrealized Gains	8,880	6,208
Held-to-Maturity, Unrealized Losses	(59)	(2,101)
Investment securities - held-to-maturity, fair value	295,833	309,335
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,850	13,861
Available-for-Sale, Unrealized Losses	(5,727)	(5,366)
Available-for-Sale securities, Fair Value	8,123	8,495
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	327,621	337,435
Available-for-Sale, Unrealized Gains	2,573	486
Available-for-Sale, Unrealized Losses	(2,975)	(6,176)
Available-for-Sale securities, Fair Value	$ 327,219	$ 331,745

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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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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