Securities Available-for-Sale - Composition 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	$ 640,036	$ 629,463
Gross Unrealized Gains	16,250	12,647
Gross Unrealized Losses	(457)	(1,837)
Fair Value	655,829us-gaap_AvailableForSaleSecurities	640,273us-gaap_AvailableForSaleSecurities
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,883	14,873
Gross Unrealized Gains	147	7
Fair Value	15,030	14,880
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	267,686	268,029
Gross Unrealized Gains	3,900	2,334
Gross Unrealized Losses	(203)	(1,078)
Fair Value	271,383	269,285
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129,010	116,897
Gross Unrealized Gains	2,419	1,257
Gross Unrealized Losses	(25)	(148)
Fair Value	131,404	118,006
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,412	45,274
Gross Unrealized Gains	1,802	1,548
Gross Unrealized Losses	(49)	(154)
Fair Value	48,165	46,668
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,701	39,834
Gross Unrealized Gains	517	232
Gross Unrealized Losses	(138)	(393)
Fair Value	40,080	39,673
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,765	136,977
Gross Unrealized Gains	5,953	5,789
Gross Unrealized Losses	(42)	(64)
Fair Value	140,676	142,702
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	632,457	621,884
Gross Unrealized Gains	14,738	11,167
Gross Unrealized Losses	(457)	(1,837)
Fair Value	646,738	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	1,512	1,480
Fair Value	$ 9,091	$ 9,059

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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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.

+ References

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-Publisher FASB

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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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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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