Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 600,744	$ 533,219	$ 454,802
Gross Unrealized Gains	11,250	10,697	7,253
Gross Unrealized Losses	(1,664)	(2,111)	(5,342)
Fair Value	610,330us-gaap_AvailableForSaleSecurities	541,805us-gaap_AvailableForSaleSecurities	456,713us-gaap_AvailableForSaleSecurities
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,954	14,953	14,948
Gross Unrealized Gains	72
Gross Unrealized Losses	(42)	(275)	(803)
Fair Value	14,984	14,678	14,145
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	154,499	137,873	110,331
Gross Unrealized Gains	4,800	3,935	2,724
Gross Unrealized Losses	(235)	(433)	(1,481)
Fair Value	159,064	141,375	111,574
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,794	10,812	10,307
Gross Unrealized Gains	193	228	285
Gross Unrealized Losses	(52)		(209)
Fair Value	10,935	11,040	10,383
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	420,497	369,581	319,216
Gross Unrealized Gains	6,185	6,534	4,244
Gross Unrealized Losses	(1,335)	(1,403)	(2,849)
Fair Value	$ 425,347	$ 374,712	$ 320,611

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

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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

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

-Publisher FASB

-Name Accounting Standards Codification

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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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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

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