Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	$ 2,531,144	$ 2,629,128
Available-for-sale, Unrealized Gains	38,470	33,309
Available-for-sale, Unrealized Losses	(24,144)	(34,606)
Available-for-Sale securities, Fair Value	2,545,470	2,627,831	2,347,766
Held-to-maturity, Amortized Cost	836,038us-gaap_HeldToMaturitySecurities	844,054us-gaap_HeldToMaturitySecurities	779,294us-gaap_HeldToMaturitySecurities
Held-to-maturity, Unrealized Gains	63,619	59,958
Held-to-maturity, Unrealized Losses	(4)	(77)
Held-to-Maturity, Fair Value	899,653	903,935	812,914
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	24,876	14,978
Available-for-sale, Unrealized Gains	302	196
Available-for-sale, Unrealized Losses		(8)
Available-for-Sale securities, Fair Value	25,178	15,166
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	707,414	692,704
Available-for-sale, Unrealized Gains	4,082	1,533
Available-for-sale, Unrealized Losses	(2,117)	(8,286)
Available-for-Sale securities, Fair Value	709,379	685,951
Held-to-maturity, Amortized Cost	166,343	167,207
Held-to-maturity, Unrealized Gains	6,293	6,279
Held-to-Maturity, Fair Value	172,636	173,486
Mortgage-backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	1,078,115	1,233,811
Available-for-sale, Unrealized Gains	19,086	18,219
Available-for-sale, Unrealized Losses	(6,470)	(10,368)
Available-for-Sale securities, Fair Value	1,090,731	1,241,662
Held-to-maturity, Amortized Cost	21,548	23,648
Held-to-maturity, Unrealized Gains	922	926
Held-to-Maturity, Fair Value	22,470	24,574
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	330,357	304,435
Available-for-sale, Unrealized Gains	11,201	11,023
Available-for-sale, Unrealized Losses	(928)	(917)
Available-for-Sale securities, Fair Value	340,630	314,541
Held-to-maturity, Amortized Cost	648,147	653,199
Held-to-maturity, Unrealized Gains	56,404	52,753
Held-to-maturity, Unrealized Losses	(4)	(77)
Held-to-Maturity, Fair Value	704,547	705,875
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	17,706	17,965
Available-for-sale, Unrealized Losses	(11,153)	(11,358)
Available-for-Sale securities, Fair Value	6,553	6,607
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	372,676	365,235
Available-for-sale, Unrealized Gains	3,799	2,338
Available-for-sale, Unrealized Losses	(3,476)	(3,669)
Available-for-Sale securities, Fair Value	$ 372,999	$ 363,904

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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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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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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Amount of accumulated unrealized loss 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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