Securities - Components of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Held To Maturity Securities [Line Items]
Held to maturity securities, Total amortized cost	$ 82,911	$ 97,519
Held to maturity securities, Gross unrealized gains	112	17
Held to maturity securities, Gross unrealized losses	(897)	(1,386)
Held to maturity securities, Fair value	82,126	96,150
Mortgage-Backed GSE Securities: Residential [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held to maturity securities, Total amortized cost	73,682	85,065
Held to maturity securities, Gross unrealized losses	(890)	(1,300)
Held to maturity securities, Fair value	72,792	83,765
States of the U.S. and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held to maturity securities, Total amortized cost	9,229	12,454
Held to maturity securities, Gross unrealized gains	112	17
Held to maturity securities, Gross unrealized losses	(7)	(86)
Held to maturity securities, Fair value	$ 9,334	$ 12,385

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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