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	Jun. 30, 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	$ 1,633,471	$ 2,017,134
Available-for-Sale, Less than 12 Months, Unrealized Losses	(24,423)	(58,667)
Available-for-Sale, 12 months or longer, Fair Value	170,324	197,213
Available-for-Sale, 12 months or longer, Unrealized Losses	(12,298)	(15,269)
Available-for-Sale, Fair Value	1,803,795	2,214,347
Available-for-Sale, Unrealized Losses	(36,721)	(73,936)
Held-to-Maturity, Less than 12 Months, Fair Value	466	59,481
Held-to-Maturity, Less than 12 Months, Unrealized Losses		(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	466	59,481
Held-to-Maturity, Unrealized Losses		(560)
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	491,338	432,192
Available-for-Sale, Less than 12 Months, Unrealized Losses	(5,762)	(12,391)
Available-for-Sale, Fair Value	491,338	432,192
Available-for-Sale, Unrealized Losses	(5,762)	(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	928,155	1,177,093
Available-for-Sale, Less than 12 Months, Unrealized Losses	(15,231)	(30,295)
Available-for-Sale, 12 months or longer, Fair Value	55,937	57,636
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,231)	(3,074)
Available-for-Sale, Fair Value	984,092	1,234,729
Available-for-Sale, Unrealized Losses	(17,462)	(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	116,598	286,351
Available-for-Sale, Less than 12 Months, Unrealized Losses	(2,461)	(13,247)
Available-for-Sale, 12 months or longer, Fair Value	5,087	4,919
Available-for-Sale, 12 months or longer, Unrealized Losses	(92)	(255)
Available-for-Sale, Fair Value	121,685	291,270
Available-for-Sale, Unrealized Losses	(2,553)	(13,502)
Held-to-Maturity, Less than 12 Months, Fair Value	466	59,481
Held-to-Maturity, Less than 12 Months, Unrealized Losses		(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	466	59,481
Held-to-Maturity, Unrealized Losses		(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	8,097	8,119
Available-for-Sale, 12 months or longer, Unrealized Losses	(8,710)	(8,892)
Available-for-Sale, Fair Value	8,097	8,119
Available-for-Sale, Unrealized Losses	(8,710)	(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	97,380	121,498
Available-for-Sale, Less than 12 Months, Unrealized Losses	(969)	(2,734)
Available-for-Sale, 12 months or longer, Fair Value	101,203	126,539
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,265)	(3,048)
Available-for-Sale, Fair Value	198,583	248,037
Available-for-Sale, Unrealized Losses	$ (2,234)	$ (5,782)

Schedule of available for sale and held to maturity securities.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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