Investment Securities - Available-for-Sale and Held-to-Maturity Investment Securities with Unrealized Losses by Aggregated Major Security Type and Length of Time in Continuous Unrealized Loss Position (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, Less than 12 Months, Fair Value	$ 1,295,822	$ 334,588
Available-for-Sale, Less than 12 Months, Unrealized Losses	(11,960)	(4,653)
Available-for-Sale, 12 months or longer, Fair Value	314,350	1,997,801
Available-for-Sale, 12 months or longer, Unrealized Losses	(9,700)	(65,920)
Available-for-Sale, Fair Value	1,610,172	2,332,389
Available-for-Sale, Unrealized Losses	(21,660)	(70,573)
Held-to-Maturity, Less than 12 Months, Fair Value		28,868
Held-to-Maturity, Less than 12 Months, Unrecognized Losses		(1,007)
Held-to-Maturity, 12 months or longer, Fair Value		261,588
Held-to-Maturity, 12 months or longer, Unrecognized Losses		(16,204)
Held-to-Maturity, Fair Value		290,456
Held-to-Maturity, Unrecognized Losses		(17,211)
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	999	3,829
Available-for-Sale, Less than 12 Months, Unrealized Losses	(2)	(12)
Available-for-Sale, 12 months or longer, Fair Value		1,472
Available-for-Sale, 12 months or longer, Unrealized Losses		(19)
Available-for-Sale, Fair Value	999	5,301
Available-for-Sale, Unrealized Losses	(2)	(31)
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	357,647	54,701
Available-for-Sale, Less than 12 Months, Unrealized Losses	(4,638)	(594)
Available-for-Sale, 12 months or longer, Fair Value		519,911
Available-for-Sale, 12 months or longer, Unrealized Losses		(10,748)
Available-for-Sale, Fair Value	357,647	574,612
Available-for-Sale, Unrealized Losses	(4,638)	(11,342)
Held-to-Maturity, 12 months or longer, Fair Value		72,359
Held-to-Maturity, 12 months or longer, Unrecognized Losses		(4,642)
Held-to-Maturity, Fair Value		72,359
Held-to-Maturity, Unrecognized Losses		(4,642)
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	786,245	82,289
Available-for-Sale, Less than 12 Months, Unrealized Losses	(6,122)	(742)
Available-for-Sale, 12 months or longer, Fair Value	212,056	1,172,984
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,930)	(43,475)
Available-for-Sale, Fair Value	998,301	1,255,273
Available-for-Sale, Unrealized Losses	(9,052)	(44,217)
Held-to-Maturity, Less than 12 Months, Fair Value		4,335
Held-to-Maturity, Less than 12 Months, Unrecognized Losses		(24)
Held-to-Maturity, 12 months or longer, Fair Value		119,207
Held-to-Maturity, 12 months or longer, Unrecognized Losses		(8,006)
Held-to-Maturity, Fair Value		123,542
Held-to-Maturity, Unrecognized Losses		(8,030)
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	120,166	99,162
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,016)	(1,340)
Available-for-Sale, 12 months or longer, Fair Value	7,006	151,097
Available-for-Sale, 12 months or longer, Unrealized Losses	(31)	(2,101)
Available-for-Sale, Fair Value	127,172	250,259
Available-for-Sale, Unrealized Losses	(1,047)	(3,441)
Held-to-Maturity, Less than 12 Months, Fair Value		24,533
Held-to-Maturity, Less than 12 Months, Unrecognized Losses		(983)
Held-to-Maturity, 12 months or longer, Fair Value		70,022
Held-to-Maturity, 12 months or longer, Unrecognized Losses		(3,556)
Held-to-Maturity, Fair Value		94,555
Held-to-Maturity, Unrecognized Losses		(4,539)
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, 12 months or longer, Fair Value	8,222	8,495
Available-for-Sale, 12 months or longer, Unrealized Losses	(5,589)	(5,366)
Available-for-Sale, Fair Value	8,222	8,495
Available-for-Sale, Unrealized Losses	(5,589)	(5,366)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	30,765	94,607
Available-for-Sale, Less than 12 Months, Unrealized Losses	(182)	(1,965)
Available-for-Sale, 12 months or longer, Fair Value	87,066	143,842
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,150)	(4,211)
Available-for-Sale, Fair Value	117,831	238,449
Available-for-Sale, Unrealized Losses	$ (1,332)	$ (6,176)