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Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, Less Than 12 Months, Market Value |  | $ 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses |  | 149
Available for sale securities, 12 Months or Greater, Market Value |  | 22,610
Available for sale securities, 12 Months or Greater, Unrealized Losses |  | 39
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, Less Than 12 Months, Market Value | $ 26,181 | 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses | 32 | 149
Available for sale securities, 12 Months or Greater, Market Value | 24,202 | 20,817
Available for sale securities, 12 Months or Greater, Unrealized Losses | $ 141 | 32
Federal Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale securities, 12 Months or Greater, Market Value |  | 1,793
Available for sale securities, 12 Months or Greater, Unrealized Losses |  | $ 7

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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