Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Less Than 12 Months, Market Value	$ 40,117	$ 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses	217	149
Available for sale securities, 12 Months or Greater, Market Value	9,973	22,610
Available for sale securities, 12 Months or Greater, Unrealized Losses	41	39
Federal Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, 12 Months or Greater, Market Value	1,798	1,793
Available for sale securities, 12 Months or Greater, Unrealized Losses	2	7
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Less Than 12 Months, Market Value	40,117	31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses	217	149
Available for sale securities, 12 Months or Greater, Market Value	8,175	20,817
Available for sale securities, 12 Months or Greater, Unrealized Losses	$ 39	$ 32

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.

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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