Changes in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Loss) Net of Taxes (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized holding loss on available for sale securities, tax benefit	$ (100)	$ 20	$ (408)	$ 185
Net Unrealized Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized holding loss on available for sale securities, tax benefit	$ (100)		$ (408)

Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration