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Marketable Investments - Summary of Company's Marketable Investments (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | $ 54,521
Gross Unrealized Losses |  | (188)
Market Value | $ 52,395 | 54,333
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 52,567 | 52,721
Gross Unrealized Gains | 1 |
Gross Unrealized Losses | (173) | (181)
Market Value | $ 52,395 | 52,540
Federal Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 1,800
Gross Unrealized Losses |  | (7)
Market Value |  | $ 1,793

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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