Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Investment Securities [Line Items]
Amortized Cost	$ 1,317,318	$ 1,350,466
Gross Unrealized Gains	16,921	15,565
Gross Unrealized Losses	(24,420)	(47,666)
Estimated Fair Value of Available for Sale	1,309,819us-gaap_AvailableForSaleSecurities	1,318,365us-gaap_AvailableForSaleSecurities
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	27,058	80
Gross Unrealized Gains	362	0
Gross Unrealized Losses	(43)	0
Estimated Fair Value of Available for Sale	27,377	80
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,682	6,693
Gross Unrealized Gains	573	328
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	7,255	7,021
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	41,926	42,040
Gross Unrealized Gains	309	0
Gross Unrealized Losses	(13,236)	(18,517)
Estimated Fair Value of Available for Sale	28,999	23,523
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,315,898	1,349,046
Gross Unrealized Gains	16,921	15,565
Gross Unrealized Losses	(24,420)	(47,666)
Estimated Fair Value of Available for Sale	1,308,399	1,316,945
Mortgage - Backed Securities - Residential [Member] \| US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	23,344	22,639
Gross Unrealized Gains	2,595	2,624
Gross Unrealized Losses	(3)	(59)
Estimated Fair Value of Available for Sale	25,936	25,204
Mortgage - Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	947,635	1,009,519
Gross Unrealized Gains	13,076	12,531
Gross Unrealized Losses	(9,830)	(27,163)
Estimated Fair Value of Available for Sale	950,881	994,887
Mortgage - Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	72	104
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	74	105
Other Government - Sponsored Enterprises [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	269,181	267,971
Gross Unrealized Gains	4	81
Gross Unrealized Losses	(1,308)	(1,927)
Estimated Fair Value of Available for Sale	267,877	266,125
Equities [Member]
Investment Securities [Line Items]
Amortized Cost	1,420	1,420
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value of Available for Sale	$ 1,420	$ 1,420

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

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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-Publisher FASB

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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