Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 5,313,209	$ 4,172,617
Available-for-Sale, Unrealized Gains	93,542	21,372
Available-for-Sale, Unrealized Losses	(21,660)	(70,573)
Available-for-Sale securities, Fair Value	5,385,091	4,123,416
Held-to-Maturity, Amortized Cost		506,334
Held-to-Maturity, Unrealized Gains		6,247
Held-to-Maturity, Unrealized Losses		(6,478)
Held-to-Maturity, Fair Value	0	506,103
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	17,567	5,332
Available-for-Sale, Unrealized Gains	117
Available-for-Sale, Unrealized Losses	(2)	(31)
Available-for-Sale securities, Fair Value	17,682	5,301
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	596,595	639,458
Available-for-Sale, Unrealized Gains	1,027	35
Available-for-Sale, Unrealized Losses	(4,638)	(11,342)
Available-for-Sale securities, Fair Value	592,984	628,151
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	3,151,550	2,243,774
Available-for-Sale, Unrealized Gains	41,363	9,738
Available-for-Sale, Unrealized Losses	(9,052)	(44,217)
Available-for-Sale securities, Fair Value	3,183,861	2,209,295
Held-to-Maturity, Amortized Cost		127,120
Held-to-Maturity, Unrealized Gains		39
Held-to-Maturity, Unrealized Losses		(2,750)
Held-to-Maturity, Fair Value		124,409
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,232,497	932,757
Available-for-Sale, Unrealized Gains	44,193	11,113
Available-for-Sale, Unrealized Losses	(1,047)	(3,441)
Available-for-Sale securities, Fair Value	1,275,643	940,429
Held-to-Maturity, Amortized Cost		305,228
Held-to-Maturity, Unrealized Gains		6,208
Held-to-Maturity, Unrealized Losses		(2,101)
Held-to-Maturity, Fair Value		309,335
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,811	13,861
Available-for-Sale, Unrealized Losses	(5,589)	(5,366)
Available-for-Sale securities, Fair Value	8,222	8,495
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	301,189	337,435
Available-for-Sale, Unrealized Gains	6,842	486
Available-for-Sale, Unrealized Losses	(1,332)	(6,176)
Available-for-Sale securities, Fair Value	$ 306,699	$ 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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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 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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