Securities - Schedule of Amortized Cost and Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	$ 1,617,439	$ 1,534,771
Securities Available For Sale, Gross Unrealized Gains	11,876	11,215
Securities Available For Sale, Gross Unrealized Losses	(10,695)	(11,921)
Securities Available For Sale, Fair Value	1,618,620	1,534,065
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	29,670	29,604
Securities Available For Sale, Gross Unrealized Gains	258	78
Securities Available For Sale, Fair Value	29,928	29,682
U.S. Government-Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	403,396	338,330
Securities Available For Sale, Gross Unrealized Gains	1,853	742
Securities Available For Sale, Gross Unrealized Losses	(1,173)	(1,939)
Securities Available For Sale, Fair Value	404,076	337,133
Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	588,103	546,572
Securities Available For Sale, Gross Unrealized Gains	6,454	7,548
Securities Available For Sale, Gross Unrealized Losses	(801)	(35)
Securities Available For Sale, Fair Value	593,756	554,085
Securities Available For Sale, Fair Value	593,756
Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	561,212	580,601
Securities Available For Sale, Gross Unrealized Gains	2,324	1,617
Securities Available For Sale, Gross Unrealized Losses	(7,939)	(9,047)
Securities Available For Sale, Fair Value	555,597	573,171
Securities Available For Sale, Fair Value	555,597
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,270	1,414
Securities Available For Sale, Gross Unrealized Gains	9	17
Securities Available For Sale, Fair Value	1,279	1,431
Securities Available For Sale, Fair Value	1,279
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	5,004	7,891
Securities Available For Sale, Gross Unrealized Gains	2
Securities Available For Sale, Gross Unrealized Losses		(11)
Securities Available For Sale, Fair Value	5,006	7,880
Securities Available For Sale, Fair Value	5,006
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	11,138	12,713
Securities Available For Sale, Gross Unrealized Gains	359	477
Securities Available For Sale, Gross Unrealized Losses	(40)	(32)
Securities Available For Sale, Fair Value	11,457	13,158
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	16,671	16,615
Securities Available For Sale, Gross Unrealized Gains	312	420
Securities Available For Sale, Gross Unrealized Losses	(742)	(857)
Securities Available For Sale, Fair Value	16,241	16,178
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities Available For Sale, Amortized Cost	1,616,464	1,533,740
Securities Available For Sale, Gross Unrealized Gains	11,571	10,899
Securities Available For Sale, Gross Unrealized Losses	(10,695)	(11,921)
Securities Available For Sale, Fair Value	1,617,340	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	305	316
Securities Available For Sale, Gross Unrealized Losses	0	0
Securities Available For Sale, Fair Value	1,280	$ 1,347
Securities Available For Sale, Fair Value	$ 1,280

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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-Publisher FASB

-Name Accounting Standards Codification

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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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