Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 843,044	$ 889,958
Gross Unrealized Gains	9,521	9,343
Gross Unrealized Losses	(9,047)	(12,331)
Fair Value	843,518us-gaap_AvailableForSaleSecurities	886,970us-gaap_AvailableForSaleSecurities
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	51,058	51,063
Gross Unrealized Gains	17	2
Gross Unrealized Losses	(602)	(1,622)
Fair Value	50,473	49,443
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	26,721	33,163
Gross Unrealized Gains	712	748
Gross Unrealized Losses	(94)	(86)
Fair Value	27,339	33,825
Obligations of States and State Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	11,101	11,160
Gross Unrealized Gains	118	0
Gross Unrealized Losses	0	(24)
Fair Value	11,219	11,136
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,331	33,340
Gross Unrealized Gains	97	127
Gross Unrealized Losses	(440)	(552)
Fair Value	32,988	32,915
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	44,432	44,500
Gross Unrealized Gains	215	127
Gross Unrealized Losses	(440)	(576)
Fair Value	44,207	44,051
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	615,880	643,382
Gross Unrealized Gains	6,081	5,854
Gross Unrealized Losses	(4,246)	(4,960)
Fair Value	617,715	644,276
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,641	23,194
Gross Unrealized Gains	0	296
Gross Unrealized Losses	(2,103)	(2,953)
Fair Value	8,538	20,537
Corporate and Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	73,241	73,585
Gross Unrealized Gains	1,712	1,645
Gross Unrealized Losses	(451)	(1,218)
Fair Value	74,502	74,012
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	21,071	21,071
Gross Unrealized Gains	784	671
Gross Unrealized Losses	(1,111)	(916)
Fair Value	$ 20,744	$ 20,826

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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-SubTopic 10

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

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-Publisher FASB

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This item represents the cost of debt and equity 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 accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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