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Investments - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized net (losses) gains on available-for-sale securities | $ 255 | $ 430
Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Maturity period of investments | One year or less |

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- Definition The change in unrealized holding gain (loss), net of tax, on available-for-sale securities included in a separate component of shareholders' equity during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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