Securities - Schedule of Amortized Cost and Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	$ 1,567,230	$ 1,534,771
Securities Available For Sale, Gross Unrealized Gains	17,045	11,215
Securities Available For Sale, Gross Unrealized Losses	(5,749)	(11,921)
Securities Available For Sale, Fair Value	1,578,526	1,534,065
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	29,704	29,604
Securities Available For Sale, Gross Unrealized Gains	300	78
Securities Available For Sale, Fair Value	30,004	29,682
U.S. Government-Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	388,428	338,330
Securities Available For Sale, Gross Unrealized Gains	2,644	742
Securities Available For Sale, Gross Unrealized Losses	(305)	(1,939)
Securities Available For Sale, Fair Value	390,767	337,133
Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	587,244	546,572
Securities Available For Sale, Gross Unrealized Gains	9,958	7,548
Securities Available For Sale, Gross Unrealized Losses	(82)	(35)
Securities Available For Sale, Fair Value	597,120	554,085
Securities Available For Sale, Fair Value	597,120
Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	529,656	580,601
Securities Available For Sale, Gross Unrealized Gains	3,220	1,617
Securities Available For Sale, Gross Unrealized Losses	(4,948)	(9,047)
Securities Available For Sale, Fair Value	527,928	573,171
Securities Available For Sale, Fair Value	527,928
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,212	1,414
Securities Available For Sale, Gross Unrealized Gains	15	17
Securities Available For Sale, Fair Value	1,227	1,431
Securities Available For Sale, Fair Value	1,227
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	4,384	7,891
Securities Available For Sale, Gross Unrealized Gains	8
Securities Available For Sale, Gross Unrealized Losses		(11)
Securities Available For Sale, Fair Value	4,392	7,880
Securities Available For Sale, Fair Value	4,392
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	10,927	12,713
Securities Available For Sale, Gross Unrealized Gains	345	477
Securities Available For Sale, Gross Unrealized Losses		(32)
Securities Available For Sale, Fair Value	11,272	13,158
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	14,700	16,615
Securities Available For Sale, Gross Unrealized Gains	267	420
Securities Available For Sale, Gross Unrealized Losses	(414)	(857)
Securities Available For Sale, Fair Value	14,553	16,178
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,566,255	1,533,740
Securities Available For Sale, Gross Unrealized Gains	16,757	10,899
Securities Available For Sale, Gross Unrealized Losses	(5,749)	(11,921)
Securities Available For Sale, Fair Value	1,577,263	1,532,718
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	975	1,031
Securities Available For Sale, Gross Unrealized Gains	288	316
Securities Available For Sale, Gross Unrealized Losses	0	0
Securities Available For Sale, Fair Value	1,263	$ 1,347
Securities Available For Sale, Fair Value	$ 1,263

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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity 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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