Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 2,834,009	$ 2,858,677
Available-for-Sale, Unrealized Gains	17,050	12,433
Available-for-Sale, Unrealized Losses	(36,721)	(73,936)
Available-for-Sale securities, Fair Value	2,814,338	2,797,174	$ 2,648,901
Held-to-Maturity, Amortized Cost	695,139	745,090	865,957
Held-to-Maturity, Unrealized Gains	54,224	39,642
Held-to-Maturity, Unrealized Losses		(560)
Held-to-Maturity, Fair Value	749,363	784,172	$ 939,855
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	5,470	6,963
Available-for-Sale, Unrealized Gains	164	140
Available-for-Sale securities, Fair Value	5,634	7,103
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	586,175	506,234
Available-for-Sale, Unrealized Gains	211	113
Available-for-Sale, Unrealized Losses	(5,762)	(12,391)
Available-for-Sale securities, Fair Value	580,624	493,956
Held-to-Maturity, Amortized Cost		40,131
Held-to-Maturity, Unrealized Gains		427
Held-to-Maturity, Fair Value		40,558
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,473,369	1,551,465
Available-for-Sale, Unrealized Gains	6,204	6,923
Available-for-Sale, Unrealized Losses	(17,462)	(33,369)
Available-for-Sale securities, Fair Value	1,462,111	1,525,019
Held-to-Maturity, Amortized Cost	8,576	10,640
Held-to-Maturity, Unrealized Gains	308	300
Held-to-Maturity, Fair Value	8,884	10,940
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	426,400	446,003
Available-for-Sale, Unrealized Gains	8,027	4,183
Available-for-Sale, Unrealized Losses	(2,553)	(13,502)
Available-for-Sale securities, Fair Value	431,874	436,684
Held-to-Maturity, Amortized Cost	686,563	694,319
Held-to-Maturity, Unrealized Gains	53,916	38,915
Held-to-Maturity, Unrealized Losses		(560)
Held-to-Maturity, Fair Value	740,479	732,674
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	16,807	17,011
Available-for-Sale, Unrealized Losses	(8,710)	(8,892)
Available-for-Sale securities, Fair Value	8,097	8,119
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	325,788	331,001
Available-for-Sale, Unrealized Gains	2,444	1,074
Available-for-Sale, Unrealized Losses	(2,234)	(5,782)
Available-for-Sale securities, Fair Value	$ 325,998	$ 326,293

Schedule of available for sale and held to maturity securities.

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

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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 debt securities categorized neither as held-to-maturity nor trading.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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

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Fair value of investments in debt securities classified as held-to-maturity.

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