Securities Available-for-Sale - Composition of Securities Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 629,463	$ 510,399
Gross Unrealized Gains	12,647	6,802
Gross Unrealized Losses	(1,837)	(7,776)
Fair Value	640,273us-gaap_AvailableForSaleSecurities	509,425us-gaap_AvailableForSaleSecurities
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,873	0
Gross Unrealized Gains	7	0
Gross Unrealized Losses	0	0
Fair Value	14,880	0
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	268,029	235,181
Gross Unrealized Gains	2,334	2,151
Gross Unrealized Losses	(1,078)	(2,581)
Fair Value	269,285	234,751
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,897	63,776
Gross Unrealized Gains	1,257	601
Gross Unrealized Losses	(148)	(603)
Fair Value	118,006	63,774
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,274	47,934
Gross Unrealized Gains	1,548	1,420
Gross Unrealized Losses	(154)	(685)
Fair Value	46,668	48,669
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,834	40,357
Gross Unrealized Gains	232	0
Gross Unrealized Losses	(393)	(1,305)
Fair Value	39,673	39,052
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	136,977	115,572
Gross Unrealized Gains	5,789	1,294
Gross Unrealized Losses	(64)	(2,602)
Fair Value	142,702	114,264
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	621,884	502,820
Gross Unrealized Gains	11,167	5,466
Gross Unrealized Losses	(1,837)	(7,776)
Fair Value	631,214	500,510
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	1,480	1,336
Gross Unrealized Losses	0	0
Fair Value	$ 9,059	$ 8,915

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

-Topic 320

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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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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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