Securities Available-for-Sale - Composition of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 658,006	$ 650,782
Gross Unrealized Gains	19,222	11,897
Gross Unrealized Losses	(7)	(1,716)
Fair Value	677,221	660,963
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,923	14,914
Gross Unrealized Gains	144	27
Gross Unrealized Losses		0
Fair Value	15,067	14,941
Obligations of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	266,807	262,045
Gross Unrealized Gains	4,733	1,825
Gross Unrealized Losses	0	(567)
Fair Value	271,540	263,303
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,502	128,458
Gross Unrealized Gains	2,828	693
Gross Unrealized Losses	0	(316)
Fair Value	137,330	128,835
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,858	39,185
Gross Unrealized Gains	1,427	1,091
Gross Unrealized Losses	0	(151)
Fair Value	39,285	40,125
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,445	69,697
Gross Unrealized Gains	1,226	183
Gross Unrealized Losses	(7)	(676)
Fair Value	70,664	69,204
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	126,892	128,904
Gross Unrealized Gains	7,490	5,988
Gross Unrealized Losses	0	(6)
Fair Value	134,382	134,886
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	650,427	643,203
Gross Unrealized Gains	17,848	9,807
Gross Unrealized Losses	(7)	(1,716)
Fair Value	668,268	651,294
Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	1,374	2,090
Gross Unrealized Losses	0	0
Fair Value	$ 8,953	$ 9,669

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)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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