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Summary of Significant Accounting Policies - Schedule of Components of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Other Comprehensive Income Loss Tax Parenthetical Disclosures [Abstract] |  |  |
Tax on Unrealized gain (loss) on investments before reclassification | $ 22 | $ (14) | $ 20

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- Definition Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities and unrealized holding gain (loss) related to transfers of securities into the available-for-sale category and out of the held-to-maturity category. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580 + Details
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- References No definition available. + Details
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