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, 2018	Dec. 31, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 334,588	$ 1,250,341
Available-for-Sale, Less than 12 Months, Unrealized Losses	(4,653)	(12,358)
Available-for-Sale, 12 months or longer, Fair Value	1,997,801	1,407,920
Available-for-Sale, 12 months or longer, Unrealized Losses	(65,920)	(51,964)
Available-for-Sale, Fair Value	2,332,389	2,658,261
Available-for-Sale, Unrealized Losses	(70,573)	(64,322)
Held-to-Maturity, Less than 12 Months, Fair Value	28,868	2,309
Held-to-Maturity, Less than 12 Months, Unrecognized Losses	(1,007)	(8)
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	2,309
Held-to-Maturity, Unrecognized Losses	(17,211)	(8)
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	3,829	1,480
Available-for-Sale, Less than 12 Months, Unrealized Losses	(12)	(5)
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	5,301	1,480
Available-for-Sale, Unrealized Losses	(31)	(5)
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	201,773
Available-for-Sale, Less than 12 Months, Unrealized Losses	(594)	(1,333)
Available-for-Sale, 12 months or longer, Fair Value	519,911	408,493
Available-for-Sale, 12 months or longer, Unrealized Losses	(10,748)	(10,027)
Available-for-Sale, Fair Value	574,612	610,266
Available-for-Sale, Unrealized Losses	(11,342)	(11,360)
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	82,289	789,804
Available-for-Sale, Less than 12 Months, Unrealized Losses	(742)	(8,692)
Available-for-Sale, 12 months or longer, Fair Value	1,172,984	774,825
Available-for-Sale, 12 months or longer, Unrealized Losses	(43,475)	(29,240)
Available-for-Sale, Fair Value	1,255,273	1,564,629
Available-for-Sale, Unrealized Losses	(44,217)	(37,932)
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	99,162	196,024
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,340)	(1,899)
Available-for-Sale, 12 months or longer, Fair Value	151,097	90,637
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,101)	(2,828)
Available-for-Sale, Fair Value	250,259	286,661
Available-for-Sale, Unrealized Losses	(3,441)	(4,727)
Held-to-Maturity, Less than 12 Months, Fair Value	24,533	2,309
Held-to-Maturity, Less than 12 Months, Unrecognized Losses	(983)	(8)
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	2,309
Held-to-Maturity, Unrecognized Losses	(4,539)	(8)
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	8,495	8,448
Available-for-Sale, 12 months or longer, Unrealized Losses	(5,366)	(8,157)
Available-for-Sale, Fair Value	8,495	8,448
Available-for-Sale, Unrealized Losses	(5,366)	(8,157)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	94,607	61,260
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,965)	(429)
Available-for-Sale, 12 months or longer, Fair Value	143,842	125,517
Available-for-Sale, 12 months or longer, Unrealized Losses	(4,211)	(1,712)
Available-for-Sale, Fair Value	238,449	186,777
Available-for-Sale, Unrealized Losses	$ (6,176)	$ (2,141)