Securities - Amortized Cost and Estimated Fair Value 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	$ 533,219	$ 488,838
Gross Unrealized Gains	10,697	4,891
Gross Unrealized Losses	(2,111)	(7,494)
Estimated Fair Value	541,805us-gaap_AvailableForSaleSecurities	486,235us-gaap_AvailableForSaleSecurities
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,953	14,947
Gross Unrealized Losses	(275)	(1,021)
Estimated Fair Value	14,678	13,926
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	137,873	112,659
Gross Unrealized Gains	3,935	2,269
Gross Unrealized Losses	(433)	(2,174)
Estimated Fair Value	141,375	112,754
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,812	10,311
Gross Unrealized Gains	228	275
Gross Unrealized Losses		(261)
Estimated Fair Value	11,040	10,325
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	369,581	349,441
Gross Unrealized Gains	6,534	2,347
Gross Unrealized Losses	(1,403)	(4,038)
Estimated Fair Value	374,712	347,750
Collateralized Debt Obligations [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost		1,480
Estimated Fair Value		$ 1,480

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 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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