Investment Securities - 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, 2017	Dec. 31, 2016
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,966,532	$ 2,017,134
Available-for-Sale, Less than 12 Months, Unrealized Losses	(46,588)	(58,667)
Available-for-Sale, 12 months or longer, Fair Value	171,379	197,213
Available-for-Sale, 12 months or longer, Unrealized Losses	(13,118)	(15,269)
Available-for-Sale, Fair Value	2,137,911	2,214,347
Available-for-Sale, Unrealized Losses	(59,706)	(73,936)
Held-to-Maturity, Less than 12 Months, Fair Value	30,153	59,481
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(129)	(560)
Held-to-Maturity, 12 months or longer, Fair Value	0	0
Held-to-Maturity, 12 months or longer, Unrealized Losses	0	0
Held-to-Maturity, Fair Value	30,153	59,481
Held-to-Maturity, Unrealized Losses	(129)	(560)
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	6,499
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1)
Available-for-Sale, Fair Value	6,499
Available-for-Sale, Unrealized Losses	(1)
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	455,813	432,192
Available-for-Sale, Less than 12 Months, Unrealized Losses	(10,279)	(12,391)
Available-for-Sale, Fair Value	455,813	432,192
Available-for-Sale, Unrealized Losses	(10,279)	(12,391)
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	1,176,954	1,177,093
Available-for-Sale, Less than 12 Months, Unrealized Losses	(24,778)	(30,295)
Available-for-Sale, 12 months or longer, Fair Value	55,246	57,636
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,713)	(3,074)
Available-for-Sale, Fair Value	1,232,200	1,234,729
Available-for-Sale, Unrealized Losses	(27,491)	(33,369)
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	227,130	286,351
Available-for-Sale, Less than 12 Months, Unrealized Losses	(9,557)	(13,247)
Available-for-Sale, 12 months or longer, Fair Value	5,702	4,919
Available-for-Sale, 12 months or longer, Unrealized Losses	(198)	(255)
Available-for-Sale, Fair Value	232,832	291,270
Available-for-Sale, Unrealized Losses	(9,755)	(13,502)
Held-to-Maturity, Less than 12 Months, Fair Value	30,153	59,481
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(129)	(560)
Held-to-Maturity, 12 months or longer, Fair Value	0	0
Held-to-Maturity, 12 months or longer, Unrealized Losses	0	0
Held-to-Maturity, Fair Value	30,153	59,481
Held-to-Maturity, Unrealized Losses	(129)	(560)
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	8,288	8,119
Available-for-Sale, 12 months or longer, Unrealized Losses	(8,564)	(8,892)
Available-for-Sale, Fair Value	8,288	8,119
Available-for-Sale, Unrealized Losses	(8,564)	(8,892)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	100,136	121,498
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,973)	(2,734)
Available-for-Sale, 12 months or longer, Fair Value	102,143	126,539
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,643)	(3,048)
Available-for-Sale, Fair Value	202,279	248,037
Available-for-Sale, Unrealized Losses	$ (3,616)	$ (5,782)