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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
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

X

- 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

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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

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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 less 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

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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 less than 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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