Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 28, 2018
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 4,834,585	$ 4,172,617
Available-for-Sale, Unrealized Gains	95,967	21,372
Available-for-Sale, Unrealized Losses	(13,573)	(70,573)
Available-for-Sale securities, Fair Value	4,916,979	4,123,416	$ 3,272,456
Held-to-Maturity, Amortized Cost	407,281	506,334	512,207
Held-to-Maturity, Unrealized Gains	18,307	6,247
Held-to-Maturity, Unrealized Losses		(6,478)
Investment securities - held-to-maturity, fair value	425,588	506,103		$ 505,566
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	5,363	5,332
Available-for-Sale, Unrealized Gains	133
Available-for-Sale, Unrealized Losses	(2)	(31)
Available-for-Sale securities, Fair Value	5,494	5,301	5,237
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	484,226	639,458
Available-for-Sale, Unrealized Gains	1,913	35
Available-for-Sale, Unrealized Losses	(35)	(11,342)
Available-for-Sale securities, Fair Value	486,104	628,151	611,696
Held-to-Maturity, Amortized Cost	33,321	73,986
Held-to-Maturity, Unrealized Gains	1,125
Held-to-Maturity, Unrealized Losses		(1,627)
Investment securities - held-to-maturity, fair value	34,446	72,359
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	3,092,991	2,243,774
Available-for-Sale, Unrealized Gains	49,135	9,738
Available-for-Sale, Unrealized Losses	(5,946)	(44,217)
Available-for-Sale securities, Fair Value	3,136,180	2,209,295	1,508,692
Held-to-Maturity, Amortized Cost	112,187	127,120
Held-to-Maturity, Unrealized Gains	3,580	39
Held-to-Maturity, Unrealized Losses		(2,750)
Investment securities - held-to-maturity, fair value	115,767	124,409
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	932,589	932,757
Available-for-Sale, Unrealized Gains	37,226	11,113
Available-for-Sale, Unrealized Losses	(340)	(3,441)
Available-for-Sale securities, Fair Value	969,475	940,429	$ 792,666
Held-to-Maturity, Amortized Cost	261,773	305,228
Held-to-Maturity, Unrealized Gains	13,602	6,208
Held-to-Maturity, Unrealized Losses		(2,101)
Investment securities - held-to-maturity, fair value	275,375	309,335
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,825	13,861
Available-for-Sale, Unrealized Losses	(6,130)	(5,366)
Available-for-Sale securities, Fair Value	7,695	8,495
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	305,591	337,435
Available-for-Sale, Unrealized Gains	7,560	486
Available-for-Sale, Unrealized Losses	(1,120)	(6,176)
Available-for-Sale securities, Fair Value	$ 312,031	$ 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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