Securities Available-for-Sale - Composition of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 650,782	$ 629,463
Gross Unrealized Gains	11,897	12,647
Gross Unrealized Losses	(1,716)	(1,837)
Fair Value	660,963	640,273
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,914	14,873
Gross Unrealized Gains	27	7
Gross Unrealized Losses	0	0
Fair Value	14,941	14,880
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	262,045	268,029
Gross Unrealized Gains	1,825	2,334
Gross Unrealized Losses	(567)	(1,078)
Fair Value	263,303	269,285
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	128,458	116,897
Gross Unrealized Gains	693	1,257
Gross Unrealized Losses	(316)	(148)
Fair Value	128,835	118,006
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,185	45,274
Gross Unrealized Gains	1,091	1,548
Gross Unrealized Losses	(151)	(154)
Fair Value	40,125	46,668
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,697	39,834
Gross Unrealized Gains	183	232
Gross Unrealized Losses	(676)	(393)
Fair Value	69,204	39,673
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	128,904	136,977
Gross Unrealized Gains	5,988	5,789
Gross Unrealized Losses	(6)	(64)
Fair Value	134,886	142,702
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	643,203	621,884
Gross Unrealized Gains	9,807	11,167
Gross Unrealized Losses	(1,716)	(1,837)
Fair Value	651,294	631,214
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	2,090	1,480
Gross Unrealized Losses	0	0
Fair Value	$ 9,669	$ 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
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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