Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 2,627,177	$ 2,424,063
Available-for-Sale, Unrealized Gains	40,888	25,766
Available-for-Sale, Unrealized Losses	(19,164)	(31,608)
Available-for-Sale securities, Fair Value	2,648,901	2,418,221	$ 2,572,521
Held-to-Maturity, Amortized Cost	865,957	872,111	823,255
Held-to-Maturity, Unrealized Gains	73,898	57,309
Held-to-Maturity, Unrealized Losses		(3)
Held-to-Maturity, Fair Value	939,855	929,417	$ 867,345
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	11,963	11,968
Available-for-Sale, Unrealized Gains	306	190
Available-for-Sale, Unrealized Losses		(8)
Available-for-Sale securities, Fair Value	12,269	12,150
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	537,728	615,578
Available-for-Sale, Unrealized Gains	3,264	1,495
Available-for-Sale, Unrealized Losses	(217)	(3,523)
Available-for-Sale securities, Fair Value	540,775	613,550
Held-to-Maturity, Amortized Cost	141,220	142,864
Held-to-Maturity, Unrealized Gains	1,928	2,899
Held-to-Maturity, Fair Value	143,148	145,763
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,323,606	1,065,936
Available-for-Sale, Unrealized Gains	15,180	10,970
Available-for-Sale, Unrealized Losses	(2,181)	(10,545)
Available-for-Sale securities, Fair Value	1,336,605	1,066,361
Held-to-Maturity, Amortized Cost	13,200	16,042
Held-to-Maturity, Unrealized Gains	523	562
Held-to-Maturity, Fair Value	13,723	16,604
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	398,727	375,671
Available-for-Sale, Unrealized Gains	18,579	11,960
Available-for-Sale, Unrealized Losses	(143)	(335)
Available-for-Sale securities, Fair Value	417,163	387,296
Held-to-Maturity, Amortized Cost	711,537	713,205
Held-to-Maturity, Unrealized Gains	71,447	53,848
Held-to-Maturity, Unrealized Losses		(3)
Held-to-Maturity, Fair Value	782,984	767,050
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	17,059	17,320
Available-for-Sale, Unrealized Losses	(10,339)	(9,420)
Available-for-Sale securities, Fair Value	6,720	7,900
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	338,094	337,590
Available-for-Sale, Unrealized Gains	3,559	1,151
Available-for-Sale, Unrealized Losses	(6,284)	(7,777)
Available-for-Sale securities, Fair Value	$ 335,369	$ 330,964

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 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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