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	Sep. 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	$ 1,576,619	$ 2,017,134
Available-for-Sale, Less than 12 Months, Unrealized Losses	(19,071)	(58,667)
Available-for-Sale, 12 months or longer, Fair Value	409,699	197,213
Available-for-Sale, 12 months or longer, Unrealized Losses	(20,273)	(15,269)
Available-for-Sale, Fair Value	1,986,318	2,214,347
Available-for-Sale, Unrealized Losses	(39,344)	(73,936)
Held-to-Maturity, Less than 12 Months, Fair Value	465	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	465	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	406,906	432,192
Available-for-Sale, Less than 12 Months, Unrealized Losses	(3,708)	(12,391)
Available-for-Sale, 12 months or longer, Fair Value	89,907
Available-for-Sale, 12 months or longer, Unrealized Losses	(3,093)
Available-for-Sale, Fair Value	496,813	432,192
Available-for-Sale, Unrealized Losses	(6,801)	(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	985,209	1,177,093
Available-for-Sale, Less than 12 Months, Unrealized Losses	(12,903)	(30,295)
Available-for-Sale, 12 months or longer, Fair Value	190,224	57,636
Available-for-Sale, 12 months or longer, Unrealized Losses	(6,859)	(3,074)
Available-for-Sale, Fair Value	1,175,433	1,234,729
Available-for-Sale, Unrealized Losses	(19,762)	(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	104,214	286,351
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,612)	(13,247)
Available-for-Sale, 12 months or longer, Fair Value	35,715	4,919
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,140)	(255)
Available-for-Sale, Fair Value	139,929	291,270
Available-for-Sale, Unrealized Losses	(2,752)	(13,502)
Held-to-Maturity, Less than 12 Months, Fair Value	465	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	465	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	8,270	8,119
Available-for-Sale, 12 months or longer, Unrealized Losses	(8,381)	(8,892)
Available-for-Sale, Fair Value	8,270	8,119
Available-for-Sale, Unrealized Losses	(8,381)	(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	80,290	121,498
Available-for-Sale, Less than 12 Months, Unrealized Losses	(848)	(2,734)
Available-for-Sale, 12 months or longer, Fair Value	85,583	126,539
Available-for-Sale, 12 months or longer, Unrealized Losses	(800)	(3,048)
Available-for-Sale, Fair Value	165,873	248,037
Available-for-Sale, Unrealized Losses	$ (1,648)	$ (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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