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	Mar. 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	$ 70,276	$ 334,588
Available-for-Sale, Less than 12 Months, Unrealized Losses	(309)	(4,653)
Available-for-Sale, 12 months or longer, Fair Value	1,941,816	1,997,801
Available-for-Sale, 12 months or longer, Unrealized Losses	(36,273)	(65,920)
Available-for-Sale, Fair Value	2,012,092	2,332,389
Available-for-Sale, Unrealized Losses	(36,582)	(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	93,188	261,588
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(3,368)	(16,204)
Held-to-Maturity, Fair Value	93,188	290,456
Held-to-Maturity, Unrecognized Losses	(3,368)	(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	4,395	3,829
Available-for-Sale, Less than 12 Months, Unrealized Losses	(2)	(12)
Available-for-Sale, 12 months or longer, Fair Value	1,481	1,472
Available-for-Sale, 12 months or longer, Unrealized Losses	(11)	(19)
Available-for-Sale, Fair Value	5,876	5,301
Available-for-Sale, Unrealized Losses	(13)	(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		54,701
Available-for-Sale, Less than 12 Months, Unrealized Losses		(594)
Available-for-Sale, 12 months or longer, Fair Value	536,114	519,911
Available-for-Sale, 12 months or longer, Unrealized Losses	(5,056)	(10,748)
Available-for-Sale, Fair Value	536,114	574,612
Available-for-Sale, Unrealized Losses	(5,056)	(11,342)
Held-to-Maturity, 12 months or longer, Fair Value	41,036	72,359
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(964)	(4,642)
Held-to-Maturity, Fair Value	41,036	72,359
Held-to-Maturity, Unrecognized Losses	(964)	(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	37,689	82,289
Available-for-Sale, Less than 12 Months, Unrealized Losses	(179)	(742)
Available-for-Sale, 12 months or longer, Fair Value	1,130,333	1,172,984
Available-for-Sale, 12 months or longer, Unrealized Losses	(22,036)	(43,475)
Available-for-Sale, Fair Value	1,168,022	1,255,273
Available-for-Sale, Unrealized Losses	(22,215)	(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	44,719	119,207
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(2,190)	(8,006)
Held-to-Maturity, Fair Value	44,719	123,542
Held-to-Maturity, Unrecognized Losses	(2,190)	(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	544	99,162
Available-for-Sale, Less than 12 Months, Unrealized Losses		(1,340)
Available-for-Sale, 12 months or longer, Fair Value	109,859	151,097
Available-for-Sale, 12 months or longer, Unrealized Losses	(596)	(2,101)
Available-for-Sale, Fair Value	110,403	250,259
Available-for-Sale, Unrealized Losses	(596)	(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	7,433	70,022
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(214)	(3,556)
Held-to-Maturity, Fair Value	7,433	94,555
Held-to-Maturity, Unrecognized Losses	(214)	(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	8,123	8,495
Available-for-Sale, 12 months or longer, Unrealized Losses	(5,727)	(5,366)
Available-for-Sale, Fair Value	8,123	8,495
Available-for-Sale, Unrealized Losses	(5,727)	(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	27,648	94,607
Available-for-Sale, Less than 12 Months, Unrealized Losses	(128)	(1,965)
Available-for-Sale, 12 months or longer, Fair Value	155,906	143,842
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,847)	(4,211)
Available-for-Sale, Fair Value	183,554	238,449
Available-for-Sale, Unrealized Losses	$ (2,975)	$ (6,176)